By the 1996 model year, all California passenger cars and trucks up to 14,000 lb. GVWR, and all Federal passenger cars and trucks up to 8,600 lb. GWVR were required to comply with the CARB-OBD II or EPA OBD requirements. The requirements applied to diesel and gasoline vehicles, and were phased in on alternative-fuel vehicles.
The California Air Resources Board (CARB) began regulating On-Board Diagnostic (OBD) systems for vehicles sold in California beginning with the 1988 model year. The initial requirements, known as OBD I, required the identification of the likely area of a fault with regard to the fuel metering system, EGR system, emission-related components and the PCM. Implementation of this new vehicle emission control monitoring regulation was done in several phases.
Changes In MIL Operation
An important change for OBD II involves when to activate the MIL. The MIL must be activated by at least the second trip if vehicle emissions could exceed 1.5 times the FTP standard. If any single component or system failure would allow the emissions to exceed this level, the MIL is activated and a related code is stored in the PCM.
Diagnostic Test Modes
The "test mode" messages available on a Scan Tool are listed below:
Differences Between OBD I & OBD II
As with OBD I, if an emission related problem is detected on a vehicle with OBD II, the MIL is activated and a code is set. However, that is the only real similarity between these systems. OBD II procedures that define emissions component and system tests, code clearing and drive cycles are more comprehensive than tests in the OBD I system.
EEC-V Hardware & Software
The EEC-V system hardware components include:
The EEC-V system software components include:
EEC-V Powertrain Controller
The purpose of the EEC-V system is to provide optimum control of the engine and transmission while meeting the objectives of the OBD II regulations. The PCM connects to various input and output devices through a wiring harness via a 104-pin connector (88 pins on the Villager and 150 pins on LS models). The PCM receives inputs from various sensors and switches, performs calculations based on data stored in an integrated circuit called Keep Alive Memory (KAM), and controls various output devices (i.e., actuators, relays, and solenoids).
Enhanced OBD Systems
Beginning in the 1994 model year, both CARB and the EPA mandated Enhanced OBD systems, commonly known as OBD II. The objectives of OBD II were to improve air quality by reducing high in-use emissions caused by emission-related faults, reduce the time between the occurrence of a fault and its detection and repair, and assist in the diagnosis and repair of an emissions-related fault.
The PCM on an EEC-V system includes a flash electrically erasable and programmable read-only memory (EEPROM) module. This software, in the form of an integrated circuit, contains the program used by the PCM to control the vehicle Powertrain. The EEPROM can be updated (reprogrammed) at a Ford dealership through the DLC and SBDS without removing the PCM. It can also be updated if it is removed and then taken to the parts counter. Changes to vehicle calibration are performed as directed by Recall Letters and Technical Service Bulletins. Refer to TSB 98-26-3 for an explanation of the complete procedure.
OBD II Systems incorporate the dedicated Ford test procedures built into the system. In effect, the Key On, Engine Off (KOEO) and Key On, Engine Running (KOER) Self-Tests are still an important functional part of the diagnostics as with earlier Ford diagnostics for OBD I systems.
Trouble codes associated with OBD II System are linked to the Ford code repair charts (Pinpoint Tests) using the customary CONT or MEM, KOEO, and KOER designators. In addition, the OBD II Main Monitors frequently run as part of the dedicated Ford Self-Tests Diagnostic Procedure
The Diagnostic Repair Chart on this page should be used as follows:
Powertrain Control Module
The PCM in the OBD II system monitors almost all Emission Control systems that affect tailpipe or evaporative emissions. In most cases, the fault must be detected before tailpipe emissions exceed 1.5 times applicable 50K or 100K-mile FTP standards. If a component exceeds emission levels or fails to operate within the design specifications, the MIL is illuminated and a code is stored within two OBD II drive cycles.
The OBD II test runs continuously or once per trip (it depends on the driving mode requirement). Tests are run once per drive cycle during specific drive patterns called trips. Codes are stored in the PCM memory when a fault is first detected. In most cases, the MIL is turned on after two trips with a fault present. If the MIL is "on", it will go off after three consecutive trips if the same fault does not reappear. If the same fault is not detected after 40 engine warmup periods, the code will be erased (Fuel and Misfire faults require 80 warmup cycles).
OBD II diagnostics require the use of a standardized Diagnostic Link Connector (DLC), standard communication protocol and messages, and standardized trouble codes and terminology. Examples of this standardization are Freeze Frame Data and I/M Readiness Monitors.
Air Injection System Monitor
The Air Injection System Monitor is an OBD diagnostic controlled by the PCM that monitors the Air Injection (AIR) system. The Oxygen Sensor Monitor must run and complete before the PCM will run this test. The PCM enables this test during AIR system operation after certain engine conditions are met and these enable criteria are met:
Air Monitor - Electric Pump Design
The AIR Monitor consists of these Solid State Monitor tests:
Air Monitor - Mechanical Pump Design
The AIR Monitor for the mechanical (belt-driven air pump) design uses two Output State Monitor configurations to perform two different circuit tests. One test is used to check for faults in the Secondary Air Bypass (AIRB) solenoid circuit. The normal function of the AIRB solenoid and valve assembly is to dump air into the atmosphere.
A second test is used to check for electrical faults in the Secondary Air Divert (AIRD) solenoid. The normal function of the AIRD solenoid and valve assembly is to direct the air either upstream or downstream.Functional Check
An AIR system functional check is done at startup with the AIR pump on or during a hot idle period if the startup part of the test was not performed. A flow test is included that uses the HO2S signal to indicate the presence of extra air injected into the exhaust stream.
Catalyst Damaging Misfire (One-Trip Detection)
If the PCM detects a Catalyst Damaging Misfire, the MIL will flash once per second within 200 engine revolutions from the point where misfire is detected. The MIL will stop flashing and remain on if the engine stops misfiring in a manner that could damage the catalyst.Catalyst Efficiency Monitor
The Catalyst Monitor is a PCM diagnostic run once per drive cycle that uses the downstream heated Oxygen Sensor (HO2S-12) to determine if a catalyst falls below a minimum level of effectiveness in its ability to control exhaust emissions. The PCM uses a program to determine the catalyst efficiency based on the oxygen storage capacity of the catalytic converter.Catalyst Monitor Graphic
Possible Causes Of A Catalyst Efficiency Fault
Catalyst Test - Calibrated Frequency Test
In Part 2 of the test a second frequency is calculated based on engine speed and load. This frequency serves as a high limit threshold for the test frequency. If the PCM detects the test frequency is less than the calibrated frequency the catalyst passes the test. If the frequency is too high, the converter or system has failed (a pending code is set).
The sequence of counting the front and rear O2S switches continues until the drive cycle completes. The ratio of total HO2S-21 switches to the total of the HO2S-11 switches is calculated. If the switch ratio is over the stored threshold, the catalyst has failed and a code is set.
Trouble Codes associated with this OBD II Monitor are listed below:
Catalyst Test - Steady State Catalyst Efficiency Test
The PCM transfers the input for closed loop fuel control from the front HO2S-11 to the rear HO2S-21 during this test. The PCM measures the output frequency of the rear HO2S. This "test frequency" indicates the current oxygen storage capacity of the converter. The slower the frequency of the test result, the higher the efficiency of the converter.Catalytic Monitor Repair Verification Trip
Start the engine, and drive in stop and go traffic for over 20 minutes. (Ambient air temperature must be over 50ºF to run this test). Drive at speeds from 25-40 mph (6 times) and then at cruise for five minutes.
The Catalyst Monitor is a diagnostic that tests the oxygen storage capacity of the catalyst. The PCM determines the capacity by comparing the switching frequency of the rear oxygen sensor to the switching frequency of the front oxygen sensor. If the catalyst is okay, the switching frequency of the rear oxygen sensor will be much slower than the frequency of the front oxygen sensor.
However, as the catalyst efficiency deteriorates its ability to store oxygen declines. This deterioration causes the rear oxygen sensor to switch more rapidly. If the PCM detects the switching frequency of the rear oxygen sensor is approaching the frequency of the front oxygen sensor, the test fails and a pending code is set. If the PCM detects a fault on consecutive trips (from two to six consecutive trips) the MIL is activated, and a trouble code is stored in the PCM memory.
The Catalyst Monitor runs after startup once a specified time has elapsed and the vehicle is in closed loop. The amount of time is subject to each PCM calibration. Certain inputs (enable criteria) from various engine sensors (i.e., CKP, ECT, IAT, TPS and VSS) are required before the Catalyst Monitor can run.
Once the Catalyst Monitor is activated, closed loop fuel control is temporarily transferred from the front oxygen sensor to the rear oxygen sensor. During the test, the Monitor analyzes the switching frequency of both sensors to determine if a catalyst has degraded.
OBD II regulations require that all emission related circuits and components controlled by the PCM that could affect emissions are monitored for circuit continuity and out-of-range faults. The Comprehensive Component Monitor (CCM) consists of four different monitoring strategies: two for inputs and two for output signals. The CCM is a two trip Monitor for emission faults on Ford vehicles.Differential Pressure Feedback EGR System
This system includes a DPFE sensor, vacuum regulator solenoid, EGR valve, orifice tube assembly, the PCM and related wiring/hoses.
When PCM strategy dictates that EGR flow should be enabled, it sends a duty cycle command to the EGR Vacuum Regulator (VR) solenoid. The VR solenoid responds by delivering a portion of its manifold vacuum signal to the EGR valve. Once the vacuum applied to the EGR valve is sufficient to overcome the EGR valve spring force, the valve opens allowing exhaust gases to enter the intake system through a metering orifice located in the orifice tube assembly. A pressure drop is created across this orifice that is proportional to the rate of EGR flow entering the intake manifold.
The DPFE sensor senses the differential pressure signal across the orifice and sends an analog signal to the PCM that indicates the rate of EGR flow. This feedback signal is used to adjust the EVR duty cycle to achieve the desired amount of EGR flow.DPFE EGR Valve - Test One
First, the Monitor tests the DPFE sensor signal to determine if it is out of normal operating range. Once this test is passed, and all enable criteria are met, the Monitor checks for a pressure differential with the engine at idle speed and with the EGR valve closed. At this point, both the upstream and downstream ports of the DPFE sensor should be reading exhaust pressure. Any differential pressure reading at this point indicates that the valve is open (and it should not be open).
Next, with the EGR valve commanded open, the differential pressure is checked. With the valve open, the sensor should signal a positive change. If there is no positive change, the downstream hose is off or plugged at the DPFE sensor. If the DPFE sensor does not indicate an upstream pressure change anytime, this indicates that it is plugged.
Next, the EGR DPFE sensor is tested with the EGR valve commanded open. If a negative DPFE sensor reading is detected, the indication is that the downstream and upstream hoses are reversed on the EGR valve. Then, the EGR valve flow is checked.
Once the engine reaches cruise speed with the ECT, MAP and TP sensor signals constant, the PCM compares the actual and expected DPFE sensor values. If the actual reading is lower than the expected value due to a restriction, the PCM fails the test and sets a code.EGR System Monitor
The EGR System Monitor is a PCM diagnostic run once per trip that monitors EGR system component functionality and components for faults that could cause vehicle tailpipe levels to exceed 1.5 times the FTP Standard. A series of sequenced tests is used to test the system.EVAP System Monitor
The EVAP System Monitor is a PCM diagnostic run once per trip that monitors the EVAP system in order to detect a loss of system integrity or leaks in the system (anywhere from 0.020" to 0.040" in diameter).
The Ford vehicles included in this article are equipped with three different EVAP systems: Purge Flow, Vapor Management Valve and On Board Refueling Vapor Recovery.EVAP Purge Monitor Test Conditions
The PCM allows canister purge to occur when the engine is warm, at wide open or part throttle (as long as the engine is not overheated). The engine can be in open or closed loop fuel control during purging.EVAP VMV Design System
This system consists of a Vapor Management Valve (VMV), fuel vapor valve, gas tank, the charcoal canister with internal atmospheric vent, related wiring and fuel vapor hoses, and the PCM. The PCM commands a normally closed (N.C.) valve "on" and "off" to control when to purge fuel vapors from the canister to the intake manifold.
Fuel vapors trapped in the sealed fuel tank are vented through a vapor valve assembly on top of fuel tank. The vapors leave the valve assembly through a single vapor line and continue on to the carbon canister for storage until they are purged to the engine for recycling.
The VMV and vent solenoid control signals are cycled "on" and "off" at a frequency of 10 hertz with a variable duty cycle. The duty cycle is ramped-up to slowly draw the canister vapors into the intake manifold.
The HO2S Monitor constantly monitors the sensor voltage and frequency. The PCM detects a high voltage condition by comparing the HO2S signal to a preset level.
A Fixed Frequency Closed Loop Test is used to check the HO2S voltage and frequency. A sample of the HO2S signal is checked to determine if the sensor is capable of switching properly or has a slow response time (referred to as a lazy sensor).Fuel System Monitor
The Fuel System Monitor is a PCM diagnostic that monitors the Adaptive Fuel Control system. The PCM uses adaptive fuel tables that are updated constantly and stored in long term memory (KAM) to compensate for wear and aging in the Fuel system components.Fuel System Monitor Operation
Once the PCM determines all the enable criteria has been are met (ECT, IAT and MAF PIDs in range and closed loop enabled), the PCM uses its adaptive strategy to "learn" changes needed to correct a Fuel system that is biased either rich or lean. The PCM accomplishes this task by monitoring Short Term and Long Term fuel trim in closed loop mode.Long & Short Term Fuel Trim
Short Term fuel trim is a PCM parameter identification (PID) used to indicate Short Term fuel adjustments. This parameter is expressed as a percentage and its range of authority is from -10% to +10%. Once the engine enters closed loop, if the PCM receives a HO2S signal that indicates the A/F mixture is richer than desired, it moves the SHRTFT command to a more negative range to correct for the rich condition.
If the PCM detects the SHRTFT is adjusting for a rich condition for too long a time, the PCM will -learn- this fact, and move LONGFT into a negative range to compensate so that SHRTFT can return to a value close to 0%. Once a change occurs to LONGFT or SHRTFT, the PCM adds a correction factor to the injector pulsewidth calculation to adjust for variations. If the change is too large, the PCM will detect a fault.
A High Emissions Misfire is set if a misfire condition is present that could cause the tailpipe emissions to exceed the FTP emissions standard by 1.5 times. If this fault is detected for two consecutive trips under similar engine speed, load and temperature conditions, the MIL is activated. It is also activated if a misfire is detected under similar conditions for two non-consecutive trips that are not 80 trips apart.Idle Air Control System Monitor
IAC Motor Test
The PCM monitors the IAC system in order to "learn" the closed loop correlation it needs to reposition the IAC solenoid (a rationality check).Input Strategies
One input strategy is used to check devices with analog inputs for opens, shorts, or out-of-range values. The CCM accomplishes this task by monitoring A/D converter input voltages. The analog inputs monitored include the ECT, IAT, MAF, TP and Transmission Range Sensors signals.
A second input strategy is used to check devices with digital and frequency inputs by performing rationality checks. The PCM uses other sensor readings and calculations to determine if a sensor or switch reading is correct under existing conditions. Some tests run continuously, some only after actuation. The inputs monitored by the PCM include the CMP, IDM, PIP, OSS, TSS and VSS signals.MIL Operation, How To Clear History Trouble Codes
If the EVAP Monitor detects a fault during a drive cycle, it will set a pending code. If it detects the fault for two consecutive trips, the MIL is activated and a code is set. The MIL will remain on for more than one trip, but will go out if conditions that caused the Monitor to fail do not reappear on three consecutive trips. After the MIL is off, the code will be erased after 40 consecutive trips if the fault does not reappear.
The Misfire Monitor is a PCM diagnostic that continuously monitors for engine misfires under all engine positive load and speed conditions (accelerating, cruising and idling). The Misfire Monitor detects misfires caused by fuel, ignition or mechanical misfire conditions. If a misfire is detected, engine conditions present at the time of the fault are written to the Freeze Frame Data. These conditions overwrite existing data.
The Misfire Monitor uses the CKP sensor signals to detect an engine misfire. The amount of contribution is calculated based upon measurements determined by crankshaft acceleration from each cylinder's power stroke.
The PCM performs various calculations to detect individual cylinder acceleration rates. If acceleration for a cylinder deviates beyond the average variation of acceleration for all cylinders, a misfire is detected.
Faults detected by the Misfire Monitor:
Misfire Monitor Operation
The Misfire Monitor is designed to measure the amount of power that each cylinder contributes to the engine. The amount of contribution is calculated based upon measurements determined by crankshaft acceleration (TDC of compression stroke to BDC of the power stroke) for each cylinder. This calculation requires accurate measurement of the crankshaft angle. Crankshaft angle measurement is determined using a low data rate system on 4-Cyl engines. The high data rate system is used to determine crankshaft angle on all other engines.
An On-Board Refueling Vapor Recovery (ORVR) system is used on late model vehicles to recover fuel vapors during vehicle refueling.
The operation of the ORVR system during refueling is described next:
An Output State Monitor in the PCM checks outputs for opens or shorts by observing the control voltage level of the related device. The control voltage is low with it on, and high with the device off. Monitored outputs include the EPC, SS1, SS2, SS3, TCC, HFC, VMV, WOT A/C Cutout and the HO2S Heater.
The Oxygen Sensor Heater Monitor is a PCM diagnostic designed to monitor the Oxygen Sensor Heater and its related circuits for faults.Front & Rear Oxygen Sensor Heater Operation
Both upstream and downstream Oxygen sensors are used on the OBD II system. These sensors are designed with additional protection around the ceramic core to protect them from condensation that could crack them if the heater is turned on with condensation present.
The HO2S heaters are not turned on until the ECT sensor signal indicates that the engine is warm. The delay period can last for as long as 5 minutes from startup. The delay allows any condensation in the Exhaust system to evaporate.
Faults detected by the HO2S or HO2S Heater Monitor:
Oxygen Sensor Heater Monitor Operation
Fuel System and Misfire Monitors must be run and complete before the PCM will start the HO2S Monitor. Additionally, parts of the HO2S Sensor Monitor are enabled during the KOER Self-Test. The HO2S Monitor is run during each drive cycle after the CKP, ECT, IAT and MAF sensor signals are within a predetermined range.
The Oxygen Sensor Heater Monitor performs its task by detecting whether the proper amount of O2 sensor voltage change occurred as the HO2S Heater is turned from "on" to "off" with the engine in closed loop. The time it takes for the HO2S-11 and HO2S-12 signal to switch (the response time) is constantly monitored by the Oxygen Sensor Monitor. Once the Oxygen Sensor Heater Monitor is enabled, if the switch time for the HO2S-11 or HO2S-12 signal is too long, the PCM fails the test, the MIL is activated and a trouble code is set.
Oxygen Sensor Monitor
The Oxygen Sensor Monitor is a PCM diagnostic designed to monitor the front and rear oxygen sensor for faults or deterioration that could cause tailpipe emissions to exceed 1.5 times the FTP standard. The front oxygen sensor voltage and response time are also monitored.
Possible Causes Of An EVAP System Failure
State Emissions Failure Misfire (Two-Trip Detection)
A State Emissions Failure Misfire is set if the misfire is sufficient to cause the vehicle to fail a State Inspection or Maintenance (I/M) Test. This fault is determined by identifying misfire percentages that would cause a -durability demonstration vehicle- to fail an Inspection Maintenance (I/M) Test. If the Misfire Monitor detects the fault for two consecutive trips with the engine at similar engine speed, load and temperature conditions, the MIL is activated and a code is set. The MIL is also activated if this type of misfire is detected under similar conditions for two non-consecutive trips of not more than 80 trips apart.
Systems & Terminology
It is very important that service technicians understand terminology related to OBD II test procedures. Several of the essential OBD II terms and definitions are discussed on the next few pages.Adaptive Fuel Control Strategy
The PCM incorporates an Adaptive Fuel Control Strategy that includes an adaptive fuel control table stored in KAM to compensate for normal changes in fuel system devices due to age or engine wear.
During closed loop operation, the Fuel System Monitor has two methods of attempting to maintain an ideal A/F ratio of 14:7 to 1 (they are referred to as short term fuel trim and long term fuel trim).
Cylinder Bank Identification
Engine sensors are identified on each engine cylinder bank as explained next.
- 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 Ford Taurus with FWD and a 3.0L V6 (VIN U) engine is shown in the Graphic on this page.
Ford vehicles equipped with OBD II use a standardized Data Link Connector (DLC). It is typically located between the left end of the instrument panel and 12 inches past vehicle centerline. The connector is mounted out of sight from vehicle passengers, but should be easy to see from outside by a technician in a kneeling position (door open). However, not all of the connectors are located in this exact area.
The DLC is rectangular in design and capable of accommodating up to 16 terminals. It has keying features to allow easy connection to the Scan Tool. Both the DLC and Scan Tool have latching features used to ensure that the Scan Tool will remain connected to the vehicle during testing.
Once the Scan Tool is connected to the DLC, it can be used to:
Diagnostic Trouble Codes
The OBD II system uses a Diagnostic Trouble Code (DTC) identification system established by the Society of Automotive Engineers (SAE) and the EPA. The first letter of a DTC is used to identify the type of computer system that has failed as shown below:
The first DTC number indicates a generic (P0xxx) or manufacturer (P1xxx) type code. A list of trouble codes is included in this section.
The number in the hundreds position indicates the specific vehicle system or subgroup that failed (i.e., P0300 for a Misfire code, P0400 for an emission system code, etc.).Drive Cycle
The term drive cycle has been used to describe a drive pattern used to verify that a trouble code, driveability symptom or intermittent fault had been fixed. With OBD II systems, this term is used to describe a vehicle drive pattern that would allow all the OBD II Monitors to initiate and run their diagnostic tests. For OBD II purposes, a minimum drive cycle includes an engine startup with continued vehicle operation that exceeds the amount of time required to enter closed loop fuel control.
The ambient or inlet air temperature must be from 40-100ºF to initiate the OBD II drive cycle. Allow the engine to warm to 130ºF prior to starting the test (except for Escort and Tracer models that require the engine be less than 100ºF to run the EVAP Monitor).
Connect the Scan Tool prior to beginning the drive cycle. Some tools are designed to emit a three-pulse beep when all of the OBD II Monitors complete their tests and DTC P1000 has been erased.
Drive Cycle Procedure
The primary intention of the Ford OBD II drive cycle is to clear a DTC P1000. The drive cycle can also be used to assist in identifying any OBD II concerns present through total Monitor testing. Perform all of the Vehicle Preparation steps. Then refer to the Drive Cycle Table and Graphic below for details on how to run a Ford OBD II Drive Cycle.
Connect a Scan Tool and have an assistant watch the Scan Tool I/M Readiness Status to determine when the Catalyst, EGR, EVAP, Fuel System, O2 Sensor, Secondary AIR and Misfire Monitors complete.
The term enable criteria describe the conditions necessary for any of the OBD II Monitors to run their diagnostic tests. Each Monitor has specific conditions that must be met before it will run its test.
Enable criteria information can be found in the vehicle manufacturer repair manuals and in this Handbook. Look under Diagnostics and then Trouble Code Conditions. This type of data can be different for each vehicle and engine type. Examples of trouble code conditions for DTC P0460 and P1168 are shown below:
|DTC||Trouble Code Title & Conditions|
|EVAP System Small Leak Conditions: Cold startup, engine running at off-idle conditions, then the PCM detected a small leak (a leak of more than 0.040") in the EVAP system.|
|FRP Sensor in Range but Low Conditions: Engine running, then the PCM detected that the FRP sensor signal was out-of-range low. Scan Tool Tip: Monitor the FRP PID for a value below 80 psi (551 kPa).|
Code information includes any of the following examples:
Freeze Frame Data
The term Freeze Frame is used to describe the engine conditions that are recorded in PCM memory at the time a Monitor detects an emissions related fault. These conditions include fuel control state, spark timing, engine speed and load.
Freeze Frame data is recorded when a system fails the first time for two-trip type faults. The Freeze Frame Data will only be overwritten by a different fault with a -higher emission priority.-
Long term fuel trim (LONGFT) is an engine parameter that indicates the amount of long term fuel adjustment made by the PCM to correct for operating conditions that vary from ideal A/F ratios. A LONGFT number that is positive (+15%) means that the HO2S is indicating a leaner than normal condition, and that it is attempting to add more fuel to the A/F mixture. If A/F ratio conditions are near ideal, the LONGFT number will be close to 0%. The PCM adjusts the LONGFT in a range from -35 to +35%. The values are in percentage on a Scan Tool.Malfunction Indicator Lamp
If the PCM detects an emission related component or system fault for two consecutive drive cycles on OBD II systems, the MIL is turned on and a trouble code is stored. The MIL is turned off if three consecutive drive cycles occur without the same fault being detected.
Most trouble codes related to a MIL are erased from KAM after 40 warmup periods if the same fault is not repeated. The MIL can be turned off after a repair by using the Scan Tool PCM Reset function.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:
Monitor Test Results
Generally, when an OBD II Monitor runs and fails a particular test during a trip, a pending code is set. If the same Monitor detects a fault for two consecutive trips, the MIL is activated and a code is set in PCM memory. The results of a particular Monitor test indicate that an emission system or component failed - not the circuit that failed!
To determine where the fault is located, follow the correct code repair chart, symptom diagnosis or intermittent test. The code and symptom repair charts are the most efficient way to repair an OBD II system.
Oxygen Sensor Identification
Oxygen sensors are identified in each cylinder bank as the front O2S (pre-catalyst) or rear O2S (post-catalyst). The acronym HO2S-11 identifies the front oxygen sensor located (Bank 1) while the HO2S-21 identifies the front oxygen sensor in Bank 2 of the engine, and so on.Short Term Fuel Trim
Short term fuel trim (SHRTFT) is an engine operating parameter that indicates the amount of short term fuel adjustment made by the PCM to compensate for operating conditions that vary from the ideal A/F ratio condition. A SHRTFT number that is negative (-15%) means that the HO2S is indicating a richer than normal condition to the PCM, and that the PCM is attempting to lean the A/F mixture. If the A/F ratio conditions are near ideal, the SHRTFT number will be close to 0%.Similar Conditions
If a pending code is set because of a Misfire or Fuel System Monitor fault, the vehicle must meet similar conditions for a second trip before the code matures the PCM activates the MIL and stores the code in memory. Refer to Note above for exceptions to this rule. The meaning of similar conditions is important when attempting to repair a fault detected by a Misfire or Fuel System Monitor.
To achieve similar conditions, the vehicle must reach the following engine running conditions simultaneously:
Summary - Similar conditions are defined as conditions that match the conditions recorded in Freeze Frame when the fault was first detected and the trouble code was set in the PCM memory.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.Trip Definition
The term OBD II Trip describes a method of driving the vehicle so that one or more of the following OBD II Monitors complete their tests:
Frequently, an emission system or component must fail a Monitor test more than once before the MIL is activated. In these cases, the first time an OBD II Monitor detects a fault during any drive cycle it sets a pending code in the PCM memory.
A pending code, which is read by selecting DDL from the Scan Tool menu, appears when Memory or Continuous codes are read. In order for a pending code to cause the MIL to activate, the original fault must be repeated under similar conditions.
This is a critical issue to understand as a pending code could remain in the PCM for a long time before the conditions that caused the code to set reappear. This type of OBD II trouble code logic is frequently referred to as the "Two-Trip Detection Logic".
The meaning of the expression warmup cycle is important. Once the fault that caused an OBD II trouble code to set is gone and the MIL is turned off, the PCM will not erase that code until after 40 warmup cycles. This is the purpose of the warmup cycle - to help clear stored codes.
However, trouble codes related to a Fuel system or Misfire fault require that 80 warmup cycles occur without the fault reappearing before codes related to these monitors will be erased from the PCM memory.
Using A Breakout Box
To use pin voltage information with a DVOM, a Breakout Box (BOB) should be installed. To connect a BOB, first turn the ignition off and remove the wire harness at the engine controller (PCM). Next, connect the correct wiring adapter to the PCM and BOB connectors. This places the BOB between the PCM and wiring so that circuit measurements can be made at the pin connections on the BOB.
In this example, the BOB is connected to the PCM connector and wire harness so that measurements can be made with a DVOM (or Lab Scope) of the Oxygen Sensor circuits with the engine running.
There are several Breakout Box designs available for use to test the PCM and its input and output circuits. However, all of them require removal of the wire harness to the PCM so that the BOB can be installed between the PCM and wire harness connector. Several breakout boxes require the use of overlays in order to allow the tool to be used on more than one year or engine type. Always verify that the correct adapter and overlays are used to prevent connection to the wrong circuits and a misdiagnosis.