S&T Series Blazer, Bravada, Envoy, Jimmy, Rainier, Trailblazer, Xtreme

OBD II System Monitors


Catalyst Degradation And Failure

When a catalyst DTC sets, remember that the converter may have internal or external damage. A few items to check are shown below:

Check for any exhaust leaks (they can hide a degraded catalyst)
Check for contaminated fuel and for miss-matched O2 sensors
Check for the use of alternate fuels and for any aftermarket parts

Catalyst Monitor

EPA regulations require that the onboard diagnostics must monitor the catalyst once per trip. The catalyst is considered degraded if the hydrocarbon levels at the tailpipe exceed the FTP standards.

The OBD II Catalyst Monitor measures oxygen storage capacity. To test the catalyst efficiency, oxygen sensors are installed before and after the Three-Way Catalyst. Voltage variations between the oxygen sensors allow the PCM to determine the performance of the catalyst.

The Catalyst Monitor test is based on the correlation conversion efficiency and oxygen storage capacity of the catalyst. An efficient catalyst will show a relatively flat output voltage on the post-catalyst heated oxygen sensor. If the catalyst is degraded, the signal from the post-catalyst HO2S will show increased activity (refer to the Graphic).

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Fig. Catalyst Monitor Graphic

Comprehensive Component Monitors

OBD II regulations require that the PCM monitor all components and systems that could cause tailpipe emissions to exceed 1.5 times the FTP Standard. The Comprehensive Component Monitor (CCM) is designed to continuously monitor components for a short-to-ground, open circuit or short-to-power condition. It also conducts rationality checks of input devices and functionality checks of output devices.

ECT Sensor Monitor

The ECT Monitor is an on-board diagnostic designed to test the ECT Sensor input for out-of-range conditions. It is also used to detect how long it takes for the engine coolant to reach closed loop temperature.

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Fig. ECT Sensor Wiring Graphic

ECT Closed Loop Enable Test

The Closed Loop Enable Test measures the amount of engine run time required for the ECT sensor input to reach closed loop threshold.

ECT Sensor Out-Of-Range Test

The ECT sensor out-of-range test monitors the temperature reading from the ECT sensor approximately every 100 milliseconds. For a fixed interval of time during the test, the PCM counts the number of ECT sensor inputs outside of its expected range. If the number of ECT sensor inputs in a high or low range exceeds a stored threshold, the ECT Sensor Monitor determines the sensor has failed high or low.

If a relatively small number of samples fall into the high range, DTC P1115 is set (ECT Sensor Intermittent High Input). If the samples fall into the low range, DTC P1114 is set (ECT Sensor Intermittent Low Input). This intermittent test is not required by the OBD II legislation. Refer to Page 3-21 for the OBD II Drive Cycle that includes this test.

EGR System Monitor

Government regulations require that the onboard diagnostics must monitor the EGR system for abnormally high or low EGR flow rates. This system is considered to have failed if a fault is detected that could change the EGR flow rate to a level below the FTP standards.

The EGR system lowers NOx emission levels caused by high engine combustion temperatures. The EGR system accomplishes this task by feeding measured amounts of exhaust gases back into the combustion chamber to change the A/F ratio and dilute the exhaust gases. This action lowers the combustion chamber temperatures.

EGR System Monitor Tests

In most cases, the EGR System Monitor detects changes in manifold pressure during EGR valve actuation to determine if the system is operating effectively. During the test, the EGR solenoid is used to force the EGR valve open during closed throttle deceleration and/or force the valve closed during steady state cruise. Manifold vacuum should increase when the valve is opened during deceleration and the vacuum should decrease when the EGR valve is closed during cruise conditions. In both cases, the amount of change in the MAP sensor input correlates to the amount of EGR flow through the valve.

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Fig. EGR System Graphic

Engine Misfire With Misfire Relief

GM introduced full-range misfire detection in 1997. This system senses misfire under all positive speed and load conditions up to vehicle "redline." Misfire is not monitored under negative speed and load conditions (deceleration). Misfire detection was only enabled under speed and load conditions to meet FTP standards before 1997.

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Fig. Misfire Monitor Test Graphic

Enhanced EVAP System

The Enhanced EVAP System includes an EVAP canister, fuel tank vapor pressure (FTVP) sensor, purge and vent solenoids, a fuel level sensor, a service port and the fuel cap. The FTVP sensor is used as part of a diagnostic strategy that is based on applying vacuum to the system and then monitoring the amount of vacuum decay over time.

Once the test criteria are met, source vacuum is used to draw a small amount of vacuum on the entire system by mechanically sealing off the designed vent path. After a calibrated amount of vacuum has been achieved, the vacuum source is turned off (system is closed). Leaks are detected by testing the amount of vacuum decay over time.

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Fig. Enhanced EVAP System Graphic

EVAP Purge System

This system includes an EVAP canister (with vapor lines), a purge solenoid (with purge lines), a vacuum switch, fuel tank and cap and related pipes and hoses. The vacuum switch is used to detect when the system is purging. It senses the flow of vacuum from the engine through the purge valve. It is closed when the system is purging.

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Fig. EVAP Purge System Graphic

EVAP System Monitor

The EVAP System Monitor is a PCM diagnostic run once per trip that monitors the Evaporative Emissions Control System in order to detect a loss of system integrity or leaks of more than 0.040" in the system.

The GM vehicles included in this section are equipped with an EVAP Purge System or an Enhanced EVAP System (1997-2002 models).

Fuel System Monitor

The Fuel System Monitor is a PCM diagnostic that continuously monitors the Fuel system to verify its ability to provide compliance with OBD II regulations. This diagnostic must fail if it detects a fault that could cause tailpipe emissions to exceed the FTP standards.

Idle Air Control System Monitor

The PCM controls the air entering into the engine with an Idle Air Control valve. To increase the idle speed, the PCM commands the pintle inside the valve away from the throttle body seat. This action allows more air to bypass through the throttle blade. To decrease the engine speed, the PCM commands the pintle towards the seat. This action reduces the amount of air bypassing the throttle blade.

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Fig. IAC System Graphic

Input Devices And The CCM

Devices that provide input signals are monitored for circuit continuity and out-of-range values including performance checks. Performance checks are defined as checks that indicate a fault when the signal from a sensor does not seem reasonable. An example would be a high TP sensor input at low engine load (e.g., with a low MAP input).

PCM Input Devices that are monitored by the CCM include:

Camshaft and Crankshaft Position Sensors
ECT, IAT, MAF, MAP, TFT and TP Sensors
Knock Sensor and Vehicle Speed Sensor

Long And Short Fuel Trim

The Fuel System Monitor is designed to monitor the averages of Long Term fuel trim (LONGFT) and Short Term Fuel Trim (SHRTFT). If the PCM determines that the fuel trim values reach and stay at their limits for too long a period of time, a fault is indicated and a code is set.

This Monitor compares the average of LONGFT values with SHRTFT values to the rich and lean limits (calibrated fail thresholds) of the test. A Pass is recorded if either value is correct with the system controlling fuel authority. If both values fall into the failure thresholds, a failure condition is exists. In if occurs a code to set and the engine conditions at that time are recorded in Freeze Frame and in the Failure Records.

The Fuel System Monitor also conducts an intrusive test to detect if a rich condition is being caused by excessive vapors from the canister.

Misfire Monitor

The Misfire Monitor is a PCM diagnostic required by OBD regulations that continuously monitors the engine for misfires under all engine positive load and speed conditions. The test operates on the principle that crankshaft rotational velocity fluctuates as each engine cylinder contributes its power. When a misfire occurs, the crankshaft speed will slow down temporarily.

The PCM monitors crankshaft rotational velocity using a crankshaft position (CKP) sensor. The CMP sensor is for cylinder identification.

Catalyst Damaging Diagnostic

This diagnostic operates when the level of misfire is sufficient to cause catalyst damage under the current operating conditions. It is designed to detect a misfire within 200-1000 crankshaft revolutions. If a Type 1-Trip code is set (or a 2-Trip code with misfire relief), the PCM will command the MIL to flash. If a catalyst-damaging misfire is no longer present, the MIL will remain on steady instead of flashing.

Emissions - Threatening Diagnostic

The Misfire test is designed to detect levels of misfire sufficient to result in emissions levels that exceed 1.5 times the FTP standard. It is designed to detect misfire within 1000-4000 crankshaft revolutions. The PCM arms the Misfire trouble code (2-Trip code) on the first trip. If the same misfire is detected on two consecutive trips, the PCM will store a History Code and illuminate the MIL. The PCM will also set this type of code and illuminate the MIL if it detects a misfire condition on non-consecutive trip if the misfire occurs under the same operating conditions (within 375 rpm of engine speed and 20% of engine load under similar engine coolant temperature) in the last 80 trips.

Misfire Counters

Whenever a misfire is detected, the Misfire Monitor counts the misfire and notes the crankshaft position at the time that the misfire occurs. These misfire counters consist of a file on each engine cylinder.

A Current and History misfire counter is maintained for each cylinder. Misfire Current Counters (Misfire Current Cyl 1-8) indicate the number of firing events out of the last 200 crankshaft revolutions that were misfires. Misfire Current Counters display real time data without a misfire code stored. Misfire History Counters (Misfire History Cyl 1-8) indicate the total number of cylinder firing events that were misfires. Misfire History counters display zero (0) until the Misfire Monitor has detected a failure and DTC P0300 is set.

Once the DTC P0300 sets, the Misfire History counters are updated every 200 revolutions of the crankshaft. If the Misfire Monitor reports a failure, the Diagnostic Executive reviews all of the Misfire counters before reporting a trouble code (to report the most current data).

Misfire Definition

Misfire is defined as lack of combustion in a cylinder due to weak compression, lack of adequate ignition spark, poor fuel metering, or any other engine mechanical, fuel or ignition system fault.

Non-Monitored Systems & Components

The PCM and CCM cannot monitor the Base Engine systems and components for faults or conditions that might cause a driveability problem. This situation can cause some confusing diagnostic situations when a Base Engine problem is present in this system.

Engine Compression

The PCM cannot detect uneven, low, or high engine cylinder compression. However, a fault in one of these areas could cause the Oxygen Sensor or Misfire Monitor to fail during testing.

Exhaust System

The PCM cannot detect a restriction or leak in the Exhaust system. However, a fault in one of these areas could cause the EGR System, Fuel System, HO2S or Misfire Monitor to fail during testing.

Fuel Injector Mechanical Fault

The PCM cannot detect if a fuel injector is restricted, stuck open or closed. However, a fault in one of these areas could result in a rich or lean condition and cause the Fuel System or HO2S Monitor to fail.

Fuel Pressure

The fuel pressure regulator controls fuel system pressure. The PCM cannot detect a restricted fuel pump inlet filter, a dirty in-line fuel filter, or a pinched fuel supply or return line. If the fuel pressure is too high or low, a fuel pressure code would not set, but a misfire or oxygen sensor code might set due to a lean or rich A/F condition.

Ignition System Secondary

The PCM cannot detect a faulty ignition coil, fouled or worn out spark plugs, ignition wires that are cross firing, or an open spark plug wire. However, the Misfire Monitor would detect these faults during testing.

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Fig. Engine Analyzer Testing the Ignition System

OBD II Monitor Tests

An OBD II Monitor is a PCM controlled (diagnostic) test run on one or more of the Emission Control systems to determine if a component or system is operating properly. As discussed, some tests either run once per trip or run continuously. The OBD II Monitor tests include:

Catalyst & Fuel System Monitors
EGR & EVAP Monitors
Misfire Monitor
Oxygen Sensor & Oxygen Sensor Heater Monitors
Secondary AIR Monitor

Output Devices And The CCM

The PCM uses an Output Device Monitor to "watch" the voltage level change on a controlled device as it is switched from "on" and "off". The eyeballs in the Graphic below represent the point at which the PCM monitors the output command signal for a change in voltage.

PCM (controlled) Output Devices monitored by the CCM include:

Air Conditioning, Cooling Fan and Fuel Pump RelaysEVAP Purge and EVAP Vent Control Solenoids
Cruise Control Vacuum and Vent Solenoids
Electronic Transaxle Controls
Idle Air Control Motor and Malfunction Indicator Lamp

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Fig. Output Device Monitor Graphic


Oxygen Sensor Heater Monitor

OBD II regulations require the Oxygen Sensor Heater Monitor to test the operation of the front and rear oxygen sensor heaters. This type of diagnostic is conducted right after a cold engine startup occurs.

HO2S Heater Test

The HO2S Heater Test measures the time required for the HO2S to become active after a cold engine startup. The time required for the oxygen sensor to become active is compared to a calibrated fault threshold in the PCM to determine the capability of the HO2S heater.

If the heater circuit has deteriorated or failed, the amount of time required for the HO2S to become active will increase. If the engine is started warm, or if a heater failure occurs during normal operation, other HO2S tests are used to detect a fault in the heater or its circuit.

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Fig. Heated Oxygen Sensor Graphic

Oxygen Sensor Monitor

OBD II regulations require that the Oxygen Sensor Monitor test the front and rear oxygen sensors for faults or deterioration that could cause tailpipe emissions to exceed 1.5 times the FTP standard. The Oxygen Sensor Monitor continuously tests the voltage output and response rate of the front and rear oxygen sensor (see the Graphic).

Fuel Control HO2S Operation

The main function of the front oxygen sensor (O2S or HO2S) is to provide the PCM with exhaust stream information to allow proper fuel control. Once it reaches operating temperature, the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust gases. Once the system is in closed loop, the PCM uses the signal from the oxygen sensor to adjust the injector pulsewidth to maintain an A/F ratio that controls tailpipe emissions and driveability.

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Fig. HO2S Response Time Graphic

Rough Road Detection

Driving over rough roads can cause engine speed variations that are similar to an engine Misfire condition and may cause false detection of a misfire. OBD II regulations allow the misfire detection test to be disabled for a short time while driving on rough road conditions.

A rough road can cause torque to be applied to the drive wheels and drive train. This torque can temporarily and intermittently decrease engine speed, and thereby cause the Misfire Monitor to incorrectly detect a misfire condition.

Secondary Air Monitor

Some GM vehicles are equipped with a Secondary Air Injection (AIR) system. OBD II regulations require that the presence of secondary airflow in the exhaust stream be monitored along with the functional monitoring of the AIR pump and its related switching valves.

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Fig. Secondary AIR System Graphic

Secondary Air Monitor Active Test

If a Passive Test fails or is inconclusive, the Active Test is started. The AIR pump is turned on during closed loop operation, and the Monitor will indicate Pass or Fail based on the HO2S-11 and SHRTFT values. A low HO2S-11 signal and increase in SHRTFT values indicates the AIR system is operating as designed.

Secondary Air Monitor Passive Test

When the Secondary AIR system is activated, excess air flows into the exhaust causing the HO2S-11 signal to drop. Then the PCM monitors the HO2S-11 signal and/or SHRTFT value for a response.

A Passive Test monitors the HO2S-11 signal after startup and also prior to closed loop operation. The AIR pump is normally enabled at this point to clean-up exhaust emissions. The HO2S-11 signal should be approximated 0-200mv if the AIR pump is delivering additional air to the exhaust during this period. The AIR Monitor will indicate a Pass if HO2S-11 indicates lean prior to closed loop operation. The Monitor also looks for the HO2S-11 to toggle as the AIR pump is disabled.