Avalanche, Sierra, Silverado, C&K Series, 1999-2005

OBD II Systems 2

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Systems & Terminology



Cylinder Bank Identification

Engine sensors are identified for each engine cylinder bank by SAE regulations as explained below.

Bank - A specific group of engine cylinders that share a common control sensor (e.g., Bank 1 identifies the location of Cyl 1 while Bank 2 identifies the cylinders on the opposite bank).

An example of the cylinder bank configuration for a Chevrolet Lumina with FWD and a 3.4L V6 (VIN X) transverse mounted engine is shown in the Graphic to the right.

Click image to see an enlarged view

Fig. Cylinder Bank Identification Graphic

Engine sensors are identified for each engine cylinder bank by SAE regulations as explained below.

Bank - A specific group of engine cylinders that share a common control sensor (e.g., Bank 1 identifies the location of Cyl 1 while Bank 2 identifies the cylinders on the opposite bank).

An example of the cylinder bank configuration for a Chevrolet Lumina with FWD and a 3.4L V6 (VIN X) transverse mounted engine is shown in the Graphic to the right.

Click image to see an enlarged view

Fig. Cylinder Bank Identification Graphic

Data Link Connector

OBD II Systems use a standardized test connector called the Data Link Connector (DLC). It is located beneath the instrument panel somewhere between the left end of the instrument panel and 12 inches (300 mm) past the vehicle centerline.

The DLC is located out of the sight of vehicle passengers, but should be easily viewable by a technician from a kneeling position outside the vehicle. The DLC is rectangular in design, capable of accommodating up to 16 terminals and has keying features to allow for easy connection. The DCL and Scan Tool connector have latching features that ensure the Scan Tool connector will remain mated when properly connected. Some common uses of the Scan Tool are:



To identify and clear any stored diagnostic trouble codes
 
To read the serial data stream information (e.g., PID data)
 
To perform Enhanced Diagnostic Tests (bi-directional Scan Tool)

Click image to see an enlarged view

Fig. Data Link Connector Graphic

 

OBD II Systems use a standardized test connector called the Data Link Connector (DLC). It is located beneath the instrument panel somewhere between the left end of the instrument panel and 12 inches (300 mm) past the vehicle centerline.

The DLC is located out of the sight of vehicle passengers, but should be easily viewable by a technician from a kneeling position outside the vehicle. The DLC is rectangular in design, capable of accommodating up to 16 terminals and has keying features to allow for easy connection. The DCL and Scan Tool connector have latching features that ensure the Scan Tool connector will remain mated when properly connected. Some common uses of the Scan Tool are:



To identify and clear any stored diagnostic trouble codes
 
To read the serial data stream information (e.g., PID data)
 
To perform Enhanced Diagnostic Tests (bi-directional Scan Tool)

Click image to see an enlarged view

Fig. Data Link Connector Graphic

 

Diagnostic Trouble Code Display

The Scan Tool can display up to five DTC options of enhanced code data on GM Vehicles They are DTC Info, Specific DTC, Freeze Frame, Fail Records (not used on all applications) and Clear Info.

The Scan Tool can display up to five DTC options of enhanced code data on GM Vehicles They are DTC Info, Specific DTC, Freeze Frame, Fail Records (not used on all applications) and Clear Info.

Conditions: To Clear Diagnostic Trouble Codes

Here are 3 methods for clearing codes from the PCM memory:



Use the Scan Tool (also clears Freeze Frame & Failure Records)
 
If battery power to the PCM is removed (battery cable or PCM fuse), all current data (DTC, Freeze Frame, Fail Records, Statistical Filters and I/M Readiness Flags) will be cleared
 
If the fault that caused a DTC to set has been corrected, the PCM will begin to count warmup cycles. Once it has counted 40 warmup cycles with no further faults detected, the DTC is cleared
 

Here are 3 methods for clearing codes from the PCM memory:



Use the Scan Tool (also clears Freeze Frame & Failure Records)
 
If battery power to the PCM is removed (battery cable or PCM fuse), all current data (DTC, Freeze Frame, Fail Records, Statistical Filters and I/M Readiness Flags) will be cleared
 
If the fault that caused a DTC to set has been corrected, the PCM will begin to count warmup cycles. Once it has counted 40 warmup cycles with no further faults detected, the DTC is cleared
 

DTC Info Mode

This mode is used to search for a specific type of stored DTC information. There are seven different selections. The repair charts may instruct the technician to test for a DTC in a particular manner.



History
 
MIL Request
 
Last Test Fail
 
Test Fail Since Codes Cleared
 
Not Run Since Codes Cleared
 
Fail This Ignition
 
DTC Status
 

This mode is used to search for a specific type of stored DTC information. There are seven different selections. The repair charts may instruct the technician to test for a DTC in a particular manner.



History
 
MIL Request
 
Last Test Fail
 
Test Fail Since Codes Cleared
 
Not Run Since Codes Cleared
 
Fail This Ignition
 
DTC Status
 

Specific DTC Mode

In this Mode, the PCM checks the status of individual diagnostic tests by their DTC number. This selection can be accessed if a DTC has passed, failed or a combination of both. Some of the individual DTC tests that are available on the Scan Tool are shown below:



Failed Last Test
 
Failed Since Clear
 
Failed This Ignition
 
History DTC
 
MIL Requested
 
Not Run Since Clear
 
Not Run This Ignition
 
Test Ran and Passed
 

In this Mode, the PCM checks the status of individual diagnostic tests by their DTC number. This selection can be accessed if a DTC has passed, failed or a combination of both. Some of the individual DTC tests that are available on the Scan Tool are shown below:



Failed Last Test
 
Failed Since Clear
 
Failed This Ignition
 
History DTC
 
MIL Requested
 
Not Run Since Clear
 
Not Run This Ignition
 
Test Ran and Passed
 

Diagnostic Trouble Codes

Each OBD II Diagnostic Trouble Code (DTC) is directly related to a particular Monitor Test. The Diagnostic Management System sets codes based on the failure of the tests during a trip (or trips). Certain tests must fail during two consecutive trips before the DTC is set.

Each OBD II Diagnostic Trouble Code (DTC) is directly related to a particular Monitor Test. The Diagnostic Management System sets codes based on the failure of the tests during a trip (or trips). Certain tests must fail during two consecutive trips before the DTC is set.

DLC Pin Assignment Table

An example of the GM DLC Pin Assignments is shown below.

CavityPin AssignmentCavityPin Assignment
1Secondary UART 8192 Baud9Primary UART
2Class II or J1850 Bus + L-Line10J1850 Bus- Line (2-wire)
3Ride Control Diagnostic11EVO or MSVA Steering
4Chassis Ground12ABS or CCM Enable
5Signal Ground13SIR Diagnostic Enable
6PCM/VCM Diagnostic Enable14E & C Bus
7ISO 9141 (K-Line) Bus15ISO 9141 (L-Line) Bus
8Keyless Entry or MRD Enable16Fused Battery Power

An example of the GM DLC Pin Assignments is shown below.

CavityPin AssignmentCavityPin Assignment
1Secondary UART 8192 Baud9Primary UART
2Class II or J1850 Bus + L-Line10J1850 Bus- Line (2-wire)
3Ride Control Diagnostic11EVO or MSVA Steering
4Chassis Ground12ABS or CCM Enable
5Signal Ground13SIR Diagnostic Enable
6PCM/VCM Diagnostic Enable14E & C Bus
7ISO 9141 (K-Line) Bus15ISO 9141 (L-Line) Bus
8Keyless Entry or MRD Enable16Fused Battery Power
Drive Cycle

General Motors OBD II systems implement the usual Strategy Based Diagnostic procedures built into the Powertrain Control Module (PCM) and Transmission Control Module (TCM). The first step in diagnosis of a problem on an OBD II system is to identify it as either a trouble code fault (Code Fault) or a driveability symptom (No Code Fault). The OBD II Drive Cycle is used to verify any repair to the system.

General Motors OBD II systems implement the usual Strategy Based Diagnostic procedures built into the Powertrain Control Module (PCM) and Transmission Control Module (TCM). The first step in diagnosis of a problem on an OBD II system is to identify it as either a trouble code fault (Code Fault) or a driveability symptom (No Code Fault). The OBD II Drive Cycle is used to verify any repair to the system.

OBD II Drive Cycle Procedure

The main intention of the OBD II Drive Cycle is to run the OBD II Main Monitors in order to determine the status of the Inspection & Maintenance (I/M) Readiness Tests. A cold engine startup (e.g., ambient air temperature of 40-100ºF) is a necessary step in preparation to run a complete OBD II Drive Cycle. In most cases the engine coolant temperature must be below 122ºF.

The main intention of the OBD II Drive Cycle is to run the OBD II Main Monitors in order to determine the status of the Inspection & Maintenance (I/M) Readiness Tests. A cold engine startup (e.g., ambient air temperature of 40-100ºF) is a necessary step in preparation to run a complete OBD II Drive Cycle. In most cases the engine coolant temperature must be below 122ºF.

OBD II Drive Pattern

The drive pattern shown below can be used to help solve:

Trouble Code Faults - Refer to the Code List (in this section) or look in electronic media or repair manuals for a code repair chart.

Driveability Symptoms & Intermittent Faults - Refer to the special repair instructions under No Code Faults in other repair manuals.

Click image to see an enlarged view

Fig. GM OBD II Drive Cycle Graphic

The drive pattern shown below can be used to help solve:

Trouble Code Faults - Refer to the Code List (in this section) or look in electronic media or repair manuals for a code repair chart.

Driveability Symptoms & Intermittent Faults - Refer to the special repair instructions under No Code Faults in other repair manuals.

Click image to see an enlarged view

Fig. GM OBD II Drive Cycle Graphic

Strategy Based Diagnostics

Strategy Based Diagnostics can be used to repair all Electrical and Electronic systems on GM vehicles. The diagnostic approach in this methodology can also be used to solve problems in an OBD II system.

Strategy Based Diagnostics can be used to repair all Electrical and Electronic systems on GM vehicles. The diagnostic approach in this methodology can also be used to solve problems in an OBD II system.

Enable Criteria

The term enable criteria is used to describe the exact Conditions: necessary for a diagnostic test to run. Each Monitor has a specific set of Conditions: that must be met before the diagnostic test is run. The enable criteria for each trouble code is listed in the column to the right of the DTC and vehicle/engine identification column under the heading Trouble Code Title & Conditions in Enable criteria is different for each Monitor test and its related code. It may include, but is not limited to the following items:



Air Conditioning (A/C) on
 
BARO, ECT, IAT, MAP, TP and Vehicle Speed Sensors
 
EVAP Canister Purge Enabled or Disabled
 
Engine Speed (RPM) and Engine Load
 
Short Term and Long Term Fuel Trim
 

The term enable criteria is used to describe the exact Conditions: necessary for a diagnostic test to run. Each Monitor has a specific set of Conditions: that must be met before the diagnostic test is run. The enable criteria for each trouble code is listed in the column to the right of the DTC and vehicle/engine identification column under the heading Trouble Code Title & Conditions in Enable criteria is different for each Monitor test and its related code. It may include, but is not limited to the following items:



Air Conditioning (A/C) on
 
BARO, ECT, IAT, MAP, TP and Vehicle Speed Sensors
 
EVAP Canister Purge Enabled or Disabled
 
Engine Speed (RPM) and Engine Load
 
Short Term and Long Term Fuel Trim
 

Failure Records

Failure Records are records in the PCM that contain the engine operating conditions present when a trouble code is stored and the MIL is illuminated. On GM vehicles, the PCM can store multiple records (i.e., the Aurora 4.0L V8 engine can store up to three records while other vehicles can store up to five records) and can update the records at any time. As each record is updated, the first record is dropped and the new failure event is recorded. These records are an enhancement of the OBD II Freeze Frame capture feature, but they can store data for any fault in memory (not just data for a MIL fault)

GM vehicles can store up to 5 Failure Records on some models. Each record is for a different code. It is also possible that there will not be Failure Records for every code if multiple codes are set. The four types of codes and their related characteristics are shown below:

Failure records store data about the operating conditions when the code was stored and the MIL was illuminated. The PCM can store multiple records and can also update these records at any time. Some vehicles can store up to 5 failure records. However, the Aurora with the Northstar 4.0L engine will only store up to 3 failure records.

The current engine operating conditions are recorded in the Failure Records buffer each time a test fails. As each record is updated, the first record is dropped and the new failure event is recorded.

The operating conditions for a diagnostic test that failed may include one or more of the following engine operating parameters:



Air Fuel Ratio
 
Airflow Rate
 
Barometric Pressure
 
Engine Load
 
Engine Coolant Temperature
 
Engine Speed
 
Fuel Trim
 
Injector Base Pulsewidth
 
Manifold Absolute Pressure
 
Open or Closed Loop Status
 
Throttle Position Angle
 
Vehicle Speed
 

Failure Records are records in the PCM that contain the engine operating conditions present when a trouble code is stored and the MIL is illuminated. On GM vehicles, the PCM can store multiple records (i.e., the Aurora 4.0L V8 engine can store up to three records while other vehicles can store up to five records) and can update the records at any time. As each record is updated, the first record is dropped and the new failure event is recorded. These records are an enhancement of the OBD II Freeze Frame capture feature, but they can store data for any fault in memory (not just data for a MIL fault)

GM vehicles can store up to 5 Failure Records on some models. Each record is for a different code. It is also possible that there will not be Failure Records for every code if multiple codes are set. The four types of codes and their related characteristics are shown below:

Failure records store data about the operating conditions when the code was stored and the MIL was illuminated. The PCM can store multiple records and can also update these records at any time. Some vehicles can store up to 5 failure records. However, the Aurora with the Northstar 4.0L engine will only store up to 3 failure records.

The current engine operating conditions are recorded in the Failure Records buffer each time a test fails. As each record is updated, the first record is dropped and the new failure event is recorded.

The operating conditions for a diagnostic test that failed may include one or more of the following engine operating parameters:



Air Fuel Ratio
 
Airflow Rate
 
Barometric Pressure
 
Engine Load
 
Engine Coolant Temperature
 
Engine Speed
 
Fuel Trim
 
Injector Base Pulsewidth
 
Manifold Absolute Pressure
 
Open or Closed Loop Status
 
Throttle Position Angle
 
Vehicle Speed
 

Flash Eeprom

GM vehicles with OBD II systems (including 1991-95 Saturn models) use a flash erasable programmable read only memory device to make running changes.

In most cases, a computer is used to download the latest changes (listed by VIN code) into a PC or the Scan Tool. The service bay PC or Scan Tool is then connected to the vehicle so that it can verify the current PROM calibration to determine if the PROM needs updating.

GM vehicles with OBD II systems (including 1991-95 Saturn models) use a flash erasable programmable read only memory device to make running changes.

In most cases, a computer is used to download the latest changes (listed by VIN code) into a PC or the Scan Tool. The service bay PC or Scan Tool is then connected to the vehicle so that it can verify the current PROM calibration to determine if the PROM needs updating.

Freeze Frame Data

Freeze Frame is an element of the Diagnostic Executive that stores engine operating conditions at the moment an emission-related fault is stored in memory (when the MIL is commanded on). This data can be used to help identify the cause of an emissions-related fault.

Regulations related to the OBD II System require that certain engine-operating conditions be captured and stored whenever the MIL is illuminated. The data captured is called Freeze Frame data. This data can be thought of as a single record of a certain set of operating conditions. Whenever the MIL is turned on, the corresponding record of operating conditions is recorded to the Freeze Frame buffer.

The Freeze Frame data can only be overwritten by data associated with a Fuel Trim or Misfire fault because data from these faults takes priority over data associated with any other type of fault. The Freeze Frame data will not be erased unless the associated History DTC is cleared.

Freeze Frame is an element of the Diagnostic Executive that stores engine operating conditions at the moment an emission-related fault is stored in memory (when the MIL is commanded on). This data can be used to help identify the cause of an emissions-related fault.

Regulations related to the OBD II System require that certain engine-operating conditions be captured and stored whenever the MIL is illuminated. The data captured is called Freeze Frame data. This data can be thought of as a single record of a certain set of operating conditions. Whenever the MIL is turned on, the corresponding record of operating conditions is recorded to the Freeze Frame buffer.

The Freeze Frame data can only be overwritten by data associated with a Fuel Trim or Misfire fault because data from these faults takes priority over data associated with any other type of fault. The Freeze Frame data will not be erased unless the associated History DTC is cleared.

Fuel Trim Diagnostics

In order to meet OBD II regulations, fuel trim information is displayed on a Scan Tool in percentages. This is different from the way fuel trim has been traditionally displayed on a Scan Tool. Short term and long term fuel trim functions within OBD II are similar to past usage, only their measurement units will differ. The fault detection logic and MIL operation is the same as that described for Misfire Diagnosis.

Click image to see an enlarged view

Fig. Fuel Trim Conversion Graphic

In order to meet OBD II regulations, fuel trim information is displayed on a Scan Tool in percentages. This is different from the way fuel trim has been traditionally displayed on a Scan Tool. Short term and long term fuel trim functions within OBD II are similar to past usage, only their measurement units will differ. The fault detection logic and MIL operation is the same as that described for Misfire Diagnosis.

Click image to see an enlarged view

Fig. Fuel Trim Conversion Graphic

Heated Oxygen Sensor

The Heated Oxygen Sensor (HO2S) detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected. The HO2S outputs a voltage from 0-1v. A value less than 0.4v indicates lean A/F ratio and a value over 0.6v indicates a rich A/F ratio.

The Heated Oxygen Sensor (HO2S) detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected. The HO2S outputs a voltage from 0-1v. A value less than 0.4v indicates lean A/F ratio and a value over 0.6v indicates a rich A/F ratio.

Explanation

In the V6 and V8 examples in the Graphic above, HO2S-11 refers to the upstream oxygen sensor and HO2S-12 refers to the downstream oxygen sensor. The downstream oxygen sensor or third oxygen is referred to as HO2S-13. The upstream HO2S-11 (or HO2S-12 on the V6 and V8 engines as noted) signal is used with the Oxygen Sensor Monitor test function. The HO2S-12 (or HO2S-13 as noted) signal is used with the Catalyst Monitor test function.

In the V6 and V8 examples in the Graphic above, HO2S-11 refers to the upstream oxygen sensor and HO2S-12 refers to the downstream oxygen sensor. The downstream oxygen sensor or third oxygen is referred to as HO2S-13. The upstream HO2S-11 (or HO2S-12 on the V6 and V8 engines as noted) signal is used with the Oxygen Sensor Monitor test function. The HO2S-12 (or HO2S-13 as noted) signal is used with the Catalyst Monitor test function.

HO2S-11, Hos-12, HO2S-13 Locations - Example 1

Throughout the manual, there are references to Heated Oxygen Sensors identified with acronyms HO2S-11, HO2S-12 and HO2S-13. In addition, the sensors are identified as either cylinder Bank 1 or cylinder Bank 2. It should be understood that Bank 1 always contains engine cylinder number 1 (Cyl 1).

Click image to see an enlarged view

Fig. HO2S Location Graphic

Throughout the manual, there are references to Heated Oxygen Sensors identified with acronyms HO2S-11, HO2S-12 and HO2S-13. In addition, the sensors are identified as either cylinder Bank 1 or cylinder Bank 2. It should be understood that Bank 1 always contains engine cylinder number 1 (Cyl 1).

Click image to see an enlarged view

Fig. HO2S Location Graphic

Heated Oxygen Sensor (Continued)
Explanation

In both of the pictures in Graphic above, HO2S-11 refers to the upstream oxygen sensor while HO2S-12 refers to the downstream oxygen sensor.

In these examples, the upstream HO2S-11 signal is used with the Oxygen Sensor Monitor test function. The downstream HO2S-12 signal is used with the Catalyst Monitor test function. HO2S location information is very important when attempting to identify the correct oxygen sensor as it relates to a trouble code repair chart.

In both of the pictures in Graphic above, HO2S-11 refers to the upstream oxygen sensor while HO2S-12 refers to the downstream oxygen sensor.

In these examples, the upstream HO2S-11 signal is used with the Oxygen Sensor Monitor test function. The downstream HO2S-12 signal is used with the Catalyst Monitor test function. HO2S location information is very important when attempting to identify the correct oxygen sensor as it relates to a trouble code repair chart.

HO2S-11, Hos2 Locations - Example 2

Throughout the manual, there are references to Heated Oxygen Sensors identified with acronyms HO2S-11, HO2S-12 and HO2S-13. In addition, the sensors are identified as either cylinder Bank 1 or cylinder Bank 2. It should be understood that Bank 1 always contains engine cylinder number 1 (Cyl 1).

Click image to see an enlarged view

Fig. HO2S Location Graphic

Throughout the manual, there are references to Heated Oxygen Sensors identified with acronyms HO2S-11, HO2S-12 and HO2S-13. In addition, the sensors are identified as either cylinder Bank 1 or cylinder Bank 2. It should be understood that Bank 1 always contains engine cylinder number 1 (Cyl 1).

Click image to see an enlarged view

Fig. HO2S Location Graphic

I/M Readiness Status

The Scan Tool can identify the Flags or I/M Readiness Status. A flag ON for a system means that the test has been run. A flag OFF for a system means that the test has not been run. If a vehicle comes in with a problem, the technician should first look at the "Flags" screen on the Scan Tool to see if all flags were set to ON. If the EGR flag is OFF, there is a possibility that the EGR system has a fault or that the EGR system tests have not have been run (the EGR system may be okay or it may have a problem). The technician needs to drive the vehicle under the trip Conditions: and get the flag set to ON before proceeding with testing the EGR system. It should be noted that OBD II trips are different for vehicles with different body codes and engines.

If power to the PCM is removed (by removing a fuse or disconnecting the battery) the Inspection & Maintenance (I/M) Flags will be reset to "off". In effect, the vehicle must be driven under specific Conditions: until all flags are set to "on".

If the power is removed, a Scan Tool can be used to do a "quick relearn step" which resets the Fuel Trim and Idle Speed to the default settings. These steps are part of the MISC, FUNCTIONAL or SPECIAL menus found on many Aftermarket Scan Tools.

The Scan Tool can identify the Flags or I/M Readiness Status. A flag ON for a system means that the test has been run. A flag OFF for a system means that the test has not been run. If a vehicle comes in with a problem, the technician should first look at the "Flags" screen on the Scan Tool to see if all flags were set to ON. If the EGR flag is OFF, there is a possibility that the EGR system has a fault or that the EGR system tests have not have been run (the EGR system may be okay or it may have a problem). The technician needs to drive the vehicle under the trip Conditions: and get the flag set to ON before proceeding with testing the EGR system. It should be noted that OBD II trips are different for vehicles with different body codes and engines.

If power to the PCM is removed (by removing a fuse or disconnecting the battery) the Inspection & Maintenance (I/M) Flags will be reset to "off". In effect, the vehicle must be driven under specific Conditions: until all flags are set to "on".

If the power is removed, a Scan Tool can be used to do a "quick relearn step" which resets the Fuel Trim and Idle Speed to the default settings. These steps are part of the MISC, FUNCTIONAL or SPECIAL menus found on many Aftermarket Scan Tools.

Malfunction Indicator Lamp

The Malfunction Indicator Lamp (MIL) looks similar to the lamp used on earlier vehicles (i.e., the Check Engine or "Service Engine Soon lamp). However, on OBD II systems, the MIL is activated under a strict set of guidelines that dictate that the lamp must be turned on when the PCM detects an emissions related fault that could impact the vehicle tailpipe emissions or Evaporative Loss system.

The Malfunction Indicator Lamp (MIL) lamp is mounted in the instrument panel. It provides several functions as described below:



To inform the driver that a fault affecting vehicle emission levels has occurred and to bring the vehicle in for service immediately.
 
To check the bulb and related circuit, the MIL will is turned on with the key in the "on" position with the engine off (KOEO). Once the engine is started, the MIL should go out.
 
If the MIL remains on with the engine running, or if a fault is suspected due to an emissions related problem, an OBD System Check of the PCM diagnostics should be performed. The tests contained in this diagnostic procedure will help find any faults that might not be detected using other diagnostic routines.

Click image to see an enlarged view

Fig. MIL Circuit Graphic

 

The Malfunction Indicator Lamp (MIL) looks similar to the lamp used on earlier vehicles (i.e., the Check Engine or "Service Engine Soon lamp). However, on OBD II systems, the MIL is activated under a strict set of guidelines that dictate that the lamp must be turned on when the PCM detects an emissions related fault that could impact the vehicle tailpipe emissions or Evaporative Loss system.

The Malfunction Indicator Lamp (MIL) lamp is mounted in the instrument panel. It provides several functions as described below:



To inform the driver that a fault affecting vehicle emission levels has occurred and to bring the vehicle in for service immediately.
 
To check the bulb and related circuit, the MIL will is turned on with the key in the "on" position with the engine off (KOEO). Once the engine is started, the MIL should go out.
 
If the MIL remains on with the engine running, or if a fault is suspected due to an emissions related problem, an OBD System Check of the PCM diagnostics should be performed. The tests contained in this diagnostic procedure will help find any faults that might not be detected using other diagnostic routines.

Click image to see an enlarged view

Fig. MIL Circuit Graphic

 

Malfunction Indicator Lamp (Continued)

Once the MIL is on, the Diagnostic Executive will turn the MIL off after three consecutive trips when test passed is recorded for the trouble code that originally caused the MIL to be activated.

If this situation occurs while the MIL is off the DTC that set when the emission-related fault occurred will be stored by the PCM in the Freeze Frame and Failure Records. The DTC will remain in memory until 40 warmup cycles (without a new fault) have been completed.

If the MIL was set due to either a Fuel system or Misfire related fault, there are other requirements to meet before the code can be cleared. Once all the requirements for these types of faults are met, the onboard diagnostics can validate that the emissions fault that caused the MIL to be activated has been corrected.

The additional requirements for a Fuel system or Misfire fault are:



Diagnostic tests that are passed must occur within 375 rpm of the engine speed data stored at the time that the last test failed
 
The engine must be at 10% of the engine load that was stored at the time the last test failed
 
Engine operating conditions must be similar to the conditions present (warmed up or warming up) when the last test failed
 

Once the MIL is on, the Diagnostic Executive will turn the MIL off after three consecutive trips when test passed is recorded for the trouble code that originally caused the MIL to be activated.

If this situation occurs while the MIL is off the DTC that set when the emission-related fault occurred will be stored by the PCM in the Freeze Frame and Failure Records. The DTC will remain in memory until 40 warmup cycles (without a new fault) have been completed.

If the MIL was set due to either a Fuel system or Misfire related fault, there are other requirements to meet before the code can be cleared. Once all the requirements for these types of faults are met, the onboard diagnostics can validate that the emissions fault that caused the MIL to be activated has been corrected.

The additional requirements for a Fuel system or Misfire fault are:



Diagnostic tests that are passed must occur within 375 rpm of the engine speed data stored at the time that the last test failed
 
The engine must be at 10% of the engine load that was stored at the time the last test failed
 
Engine operating conditions must be similar to the conditions present (warmed up or warming up) when the last test failed
 

Intermittent MIL -On- Conditions:

If the PCM detects a fault and then the fault goes away, the MIL will remain on until after three trips are completed without the same fault reoccurring. This type of MIL "on" condition could appear to be an intermittent fault, but most OBD II faults are not intermittent in nature.

An example of an intermittent MIL condition is described next. If the customer were to leave the fuel filler cap loose or off, and the vehicle was driven under the correct code conditions (meeting all enable criteria for an EVAP large leak trouble code), the PCM would turn the MIL on the second time it ran the EVAP Monitor and failed the test. The MIL would remain on until the vehicle was refueled and the customer tightened the fuel cap properly or noticed it was off. Once the vehicle was driven under the correct EVAP large leak code conditions, the EVAP Monitor would run the test. If the EVAP test passed for three consecutive trips, the PCM would turn off the MIL (this is true for some vehicles depending upon the model year).

However, in this case, the related trouble code would remain in memory until 40 OBD II warmup cycles occur (80 OBD II warmup cycles for Fuel system and Misfire faults) without the fault reoccurring. If the vehicle was brought in for service and a PCM Reset step was done, the MIL would then go out and the codes would clear.

If the PCM detects a fault and then the fault goes away, the MIL will remain on until after three trips are completed without the same fault reoccurring. This type of MIL "on" condition could appear to be an intermittent fault, but most OBD II faults are not intermittent in nature.

An example of an intermittent MIL condition is described next. If the customer were to leave the fuel filler cap loose or off, and the vehicle was driven under the correct code conditions (meeting all enable criteria for an EVAP large leak trouble code), the PCM would turn the MIL on the second time it ran the EVAP Monitor and failed the test. The MIL would remain on until the vehicle was refueled and the customer tightened the fuel cap properly or noticed it was off. Once the vehicle was driven under the correct EVAP large leak code conditions, the EVAP Monitor would run the test. If the EVAP test passed for three consecutive trips, the PCM would turn off the MIL (this is true for some vehicles depending upon the model year).

However, in this case, the related trouble code would remain in memory until 40 OBD II warmup cycles occur (80 OBD II warmup cycles for Fuel system and Misfire faults) without the fault reoccurring. If the vehicle was brought in for service and a PCM Reset step was done, the MIL would then go out and the codes would clear.

MIL On/Off Guidelines

If an emission-related fault is detected, the Diagnostic Executive will activate the MIL and allow it to remain "on" until the system or component passes the same test for three consecutive trips without the fault reoccurring.

If an emission-related fault is detected, the Diagnostic Executive will activate the MIL and allow it to remain "on" until the system or component passes the same test for three consecutive trips without the fault reoccurring.

MIL On Or Flashing - Fuel System Or Misfire Conditions:

If a Fuel system problem or misfire condition is present that could damage the catalyst, the MIL will flash once per second. The MIL will continue to flash until the vehicle is outside of the speed and load conditions that could cause possible catalyst damage. It will stop flashing and remain on once these conditions: are no longer present.

If a Fuel system problem or misfire condition is present that could damage the catalyst, the MIL will flash once per second. The MIL will continue to flash until the vehicle is outside of the speed and load conditions that could cause possible catalyst damage. It will stop flashing and remain on once these conditions: are no longer present.

Misfire Diagnostics

The Diagnostic Executive has the capability of alerting the driver of potentially damaging levels of engine misfire that could damage the catalyst. If this type of condition is detected, the MIL is commanded to flash on/off once per second during the actual misfire condition.

The Misfire Diagnostics represents a special type of trouble code diagnostics. Each time a misfire is detected, the engine load, engine speed and coolant temperature at that moment are recorded and the last reported set of Conditions: are stored in Freeze Frame when the key is turned off. During the next few key on/off cycles, the stored Conditions: are used as a reference for similar conditions.

If an emissions threatening misfire occurs for two consecutive trips, the PCM treats the fault as a normal Type B trouble code (it turns on the MIL and stores a code). However if a misfire is detected on two non-consecutive trips, the stored Conditions: are compared with the current conditions. If a misfire occurs on a second non-consecutive trip under the similar Conditions: listed below, the MIL is illuminated:



The engine load conditions are within 10% of the previous failure
 
Engine speed is within 375 rpm of the previous failure
 
Engine temperature is in the same range as the previous failure
 

The Diagnostic Executive has the capability of alerting the driver of potentially damaging levels of engine misfire that could damage the catalyst. If this type of condition is detected, the MIL is commanded to flash on/off once per second during the actual misfire condition.

The Misfire Diagnostics represents a special type of trouble code diagnostics. Each time a misfire is detected, the engine load, engine speed and coolant temperature at that moment are recorded and the last reported set of Conditions: are stored in Freeze Frame when the key is turned off. During the next few key on/off cycles, the stored Conditions: are used as a reference for similar conditions.

If an emissions threatening misfire occurs for two consecutive trips, the PCM treats the fault as a normal Type B trouble code (it turns on the MIL and stores a code). However if a misfire is detected on two non-consecutive trips, the stored Conditions: are compared with the current conditions. If a misfire occurs on a second non-consecutive trip under the similar Conditions: listed below, the MIL is illuminated:



The engine load conditions are within 10% of the previous failure
 
Engine speed is within 375 rpm of the previous failure
 
Engine temperature is in the same range as the previous failure
 

Monitor Software

The Diagnostic Executive contains software designed to allow the PCM to organize and prioritize the Main Monitor tests and procedures, and to record and display test results and diagnostic trouble codes. The functions controlled by this software include:



To control the diagnostic system so that the vehicle will continue to operate in a normal manner during testing.
 
To ensure that all of the OBD II Monitors run during the first two sample periods of the Federal Test Procedure.
 
To ensure that all OBD II Monitors and their related tests are sequenced so that required inputs (enable criteria) for a particular Monitor are present prior to running that particular Monitor.
 
To sequence the running of the Monitors to eliminate the possibility of different Monitor tests interfering with each other or upsetting normal vehicle operation.
 
To provide a Scan Tool interface by coordinating the operation of special tests or data requests.
 

The Diagnostic Executive contains software designed to allow the PCM to organize and prioritize the Main Monitor tests and procedures, and to record and display test results and diagnostic trouble codes. The functions controlled by this software include:



To control the diagnostic system so that the vehicle will continue to operate in a normal manner during testing.
 
To ensure that all of the OBD II Monitors run during the first two sample periods of the Federal Test Procedure.
 
To ensure that all OBD II Monitors and their related tests are sequenced so that required inputs (enable criteria) for a particular Monitor are present prior to running that particular Monitor.
 
To sequence the running of the Monitors to eliminate the possibility of different Monitor tests interfering with each other or upsetting normal vehicle operation.
 
To provide a Scan Tool interface by coordinating the operation of special tests or data requests.
 

Similar Conditions

For Fuel System and Misfire codes, the engine operating conditions must be similar to conditions present when the fault was first detected to clear a code. In effect, the engine load must be within 10%, engine speed within 375 rpm and the engine temperature similar (cold or warm) before the Fuel System or Misfire Monitor will retest for a code.

For Fuel System and Misfire codes, the engine operating conditions must be similar to conditions present when the fault was first detected to clear a code. In effect, the engine load must be within 10%, engine speed within 375 rpm and the engine temperature similar (cold or warm) before the Fuel System or Misfire Monitor will retest for a code.

Standard Corporate Protocol

On vehicles equipped with OBD II, a Standard Corporate Protocol (SCP) communication language is used to exchange bi-directional messages between stand-alone modules and devices. With this type of system, two or more messages can be sent over one circuit.

On vehicles equipped with OBD II, a Standard Corporate Protocol (SCP) communication language is used to exchange bi-directional messages between stand-alone modules and devices. With this type of system, two or more messages can be sent over one circuit.

Trip Definition

An OBD II trip is official when all the enable criteria for a given test (Monitor) are met. Because enable criteria vary from one Monitor to another, the definition of a trip varies as well. The trip requirements (criteria) can include seemingly unrelated items such as driving style, the length of the trip, and ambient temperature. A minimum requirement for a trip includes one key cycle, and in most cases, the engine must run for a period of time before the test is enabled.

Vehicle tests vary in length - some are performed only once per trip and some are performed continuously. The Catalyst, EGR, EVAP and Oxygen Sensor tests are performed once per trip. The Component Monitor, Fuel and Misfire tests are performed continuously. An OBD II trip is defined as a "key on-drive-the-vehicle-key-off" cycle in which the vehicle is operated in a manner that satisfies the criteria for a test.

An OBD II trip is official when all the enable criteria for a given test (Monitor) are met. Because enable criteria vary from one Monitor to another, the definition of a trip varies as well. The trip requirements (criteria) can include seemingly unrelated items such as driving style, the length of the trip, and ambient temperature. A minimum requirement for a trip includes one key cycle, and in most cases, the engine must run for a period of time before the test is enabled.

Vehicle tests vary in length - some are performed only once per trip and some are performed continuously. The Catalyst, EGR, EVAP and Oxygen Sensor tests are performed once per trip. The Component Monitor, Fuel and Misfire tests are performed continuously. An OBD II trip is defined as a "key on-drive-the-vehicle-key-off" cycle in which the vehicle is operated in a manner that satisfies the criteria for a test.

Type 'A' Codes

Type 'A' Codes are emissions-related



Request the MIL "on" the first trip a fault is detected
 
Store a History Code on first trip a fault is detected
 
Store Freeze Frame data the first trip a fault is detected (if empty)
 
Store a Failure Record
 
Update the Failure Record each time a Monitor Test fails
 

Type 'A' Codes are emissions-related



Request the MIL "on" the first trip a fault is detected
 
Store a History Code on first trip a fault is detected
 
Store Freeze Frame data the first trip a fault is detected (if empty)
 
Store a Failure Record
 
Update the Failure Record each time a Monitor Test fails
 

Type 'B' Codes

Type 'B' Codes are emissions-related



Are "armed" or pending after one trip when a fault is detected
 
Are "disarmed" after 1-Trip if the second consecutive trip passes
 
Request the MIL "on" the 2nd consecutive trip if the fault occurs
 
Set a History Code on the 2nd consecutive trip if the fault occurs
 
Store Freeze Frame Data the 1st trip that a fault is detected
 
Store a Failure Record the first trip a fault is detected
 
Update a Failure Record the first time the test fails each key cycle
 

Type 'B' Codes are emissions-related



Are "armed" or pending after one trip when a fault is detected
 
Are "disarmed" after 1-Trip if the second consecutive trip passes
 
Request the MIL "on" the 2nd consecutive trip if the fault occurs
 
Set a History Code on the 2nd consecutive trip if the fault occurs
 
Store Freeze Frame Data the 1st trip that a fault is detected
 
Store a Failure Record the first trip a fault is detected
 
Update a Failure Record the first time the test fails each key cycle
 

Type 'C' Codes (Changed To Type C1 In Mid-1997)

Type 'C' Codes are non-emissions related



Request the SES (lamp) or DIC the first time the fault is detected
 
Store a History Code the first time the fault is detected
 
Do not store a Freeze Frame Record
 
Store a Failure Record the first time the test fails each key
 
Update a Failure Record the first time a test fails each key cycle
 

Type 'C' Codes are non-emissions related



Request the SES (lamp) or DIC the first time the fault is detected
 
Store a History Code the first time the fault is detected
 
Do not store a Freeze Frame Record
 
Store a Failure Record the first time the test fails each key
 
Update a Failure Record the first time a test fails each key cycle
 

Type 'D' Codes (Changed To Type C0 In Mid-1997)

Type 'D' Codes are non-emission related



Do not request the PCM or other controller to turn on a lamp
 
Store a History Code on the first trip that a fault is detected
 
Do not store a Freeze Frame Record
 
Store a Failure Record if a non-emission related test fails
 
Update the Failure Record each time a non-emissions test fails
 

Type 'D' Codes are non-emission related



Do not request the PCM or other controller to turn on a lamp
 
Store a History Code on the first trip that a fault is detected
 
Do not store a Freeze Frame Record
 
Store a Failure Record if a non-emission related test fails
 
Update the Failure Record each time a non-emissions test fails
 

Warm-up Cycle

Once the MIL is off, a stored trouble code will remain in memory until 40 warmup cycles are completed without the fault occurring.

A warmup cycle is defined as a trip that includes a change in engine temperature of at least 40ºF and where it reaches 160ºF.

Click image to see an enlarged view

Fig. OBD II Warmup Graphic

Once the MIL is off, a stored trouble code will remain in memory until 40 warmup cycles are completed without the fault occurring.

A warmup cycle is defined as a trip that includes a change in engine temperature of at least 40ºF and where it reaches 160ºF.

Click image to see an enlarged view

Fig. OBD II Warmup Graphic

UART Serial Data



There are two methods of data transmission used on OBD II systems. One method is the Universally Asynchronous Receiving/Transmitting (UART) protocol. UART is an interface device that allows a controller to send and receive serial data. Serial data refers to information that is transferred in a linear fashion (over a single line) one bit at a time. A data bus describes the electronic pathway through which serial data travels. UART receives serial data, converts the data to parallel format, and then places it on the data bus for use by the vehicle computer. UART can also convert data in parallel format to serial format, and then transmit the converted data to the Scan Tool.

There are two methods of data transmission used on OBD II systems. One method is the Universally Asynchronous Receiving/Transmitting (UART) protocol. UART is an interface device that allows a controller to send and receive serial data. Serial data refers to information that is transferred in a linear fashion (over a single line) one bit at a time. A data bus describes the electronic pathway through which serial data travels. UART receives serial data, converts the data to parallel format, and then places it on the data bus for use by the vehicle computer. UART can also convert data in parallel format to serial format, and then transmit the converted data to the Scan Tool.

Vehicle Applications



C/K Body Codes

1999-2005 C/K Series, Sierra, Silverado,



Engine: 4.3L V6 VIN W
 
Engine: 4.8L V8 VIN V
 
Engine: 5.0L V8 VIN M
 
Engine: 5.3L V8 VIN T
 
Engine: 5.3L V8 Flexible Fuel VIN Z
 
Engine: 5.7L V8 (GAS/CNG) VIN K
 
Engine: 5.7L V8 VIN R
 
Engine: 6.0L V8 VIN N, U
 
Engine: 6.5L V8 Diesel (1999-2000) VIN F
 
Engine: 6.5L V8 Turbo Diesel (1999-2000) VIN S
 
Engine: 6.6L V8 Turbo Diesel (2001-05) VIN 1, J
 
Engine: 7.2L V8 Diesel VIN C
 
Engine: 7.4L V8 VIN J
 
Engine: 8.1L V8 VIN E, G
 

C/K Body Codes

1996-2005 C/K Series, Escalade, Suburban, Tahoe, Yukon 2 & 4-Door Utility



Engine: 4.3L V6 VIN W
 
Engine: 4.8L V8 VIN V
 
Engine: 5.0L V8 VIN M
 
Engine: 5.3L V8 VIN T
 
Engine: 5.3L V8 Flexible Fuel VIN Z
 
Engine: 5.7L V8 (GAS/CNG) VIN K
 
Engine: 5.7L V8 VIN R
 
Engine: 6.0L V8 VIN N, U
 
Engine: 6.5L V8 Diesel (1996-2000) VIN F
 
Engine: 6.5L V8 Turbo Diesel (1997-98) VIN P
 
Engine: 6.5L V8 Turbo Diesel (1997-2000) VIN S
 
Engine: 6.6L V8 Turbo Diesel (2001-02) VIN 1, J
 
Engine: 7.2L V8 Diesel VIN C
 
Engine: 7.4L V8 VIN J
 
Engine: 8.1L V8 VIN E, G
 

 
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