Powertrain Controls Diagnosis

Diagnostic Chart:

Wiring Diagram:

Circuit Description
In order to provide the best possible combination of driveability, fuel economy, and emission control, the vehicle uses a Closed Loop air and fuel metering system. While in a Closed Loop, the VCM monitors the oxygen sensor signal voltage. The VCM adjusts the fuel delivery based on a signal voltage. The long and short term fuel values, which a scan tool can monitor, indicates a change made to the fuel delivery. Ideal fuel trim values are around 128; if the oxygen sensor signal indicates a lean condition, the VCM adds fuel. This results in fuel trim values above 128. If The oxygen sensor detects a rich condition, the fuel trim values will read below 128. This indicates that the VCM is reducing the amount of fuel delivered. If an excessively rich condition is detected, the VCM sets this Diagnostic Trouble Code (DTC) which is a type B DTC.

Conditions for Setting the DTC

^ No IAC or idle DTCs.
^ No HO2S sensor DTCs.
^ No TP sensor DTCs.
^ No MAP sensor DTCs.
^ No EGR sensor DTCs.
^ No EVAP sensor DTCs.
^ No ECT sensor DTCs.
^ No MAF sensor DTCs.
^ No IAT sensor DTCs.
^ No VS sensor DTCs.
^ System voltage is greater than 10.V but less than 17.1V.
^ No Misfire DTCs.
^ Throttle position less than 69.9%.
^ Engine speed greater than 525 RPM but less than 4500 RPM.
^ The BARO greater than 70 kPa.
^ The ECT greater than 0°C but less than 99.7°C.
^ The MAP is greater than 20 kPa but less than 98.9 kPa.
^ The IAT greater than -20°C but less than 69.7°C.
^ The air flow greater than 3 grams per second but less than 150 grams per second.
^ The vehicle speed less than 85 mph.

Action Taken When the DTC Sets

Important: To complete a trip cycle for the fuel trim diagnostic, the fuel trim diagnostic test must enable and run according to the current DTC Will Set When criteria above. A trip is not considered complete until the fuel trim diagnostic has run by the VCM.

When the current DTC initially sets, a software flag indicating an emissions related fault sets for 3 trips. The flag will clear provided the lean condition is no longer present. However, if the VCM detects the conditions for setting the current DTC during a trip while the flag is still set, the VCM turns on the Malfunction Indicator Lamp (MIL). The MIL remains on for all the subsequent trips until the DTC has been cleared by service or the lean condition has not been detected and the conditions for checking the fuel trim have been met for 3 consecutive trips.

Conditions for Clearing the MIL/DTC
The VCM turns the MIL off after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°F from the start-up coolant temperature and the engine coolant temperature exceeds 160°F during that same ignition cycle) or the scan tool clearing feature has been used.

Diagnostic Aids
If using the diagnostic tables does not isolate the problem, try monitoring the L.T. fuel trim and fuel trim cell while operating the vehicle under various loads. This may isolate the condition which caused the setting of the current DTC.

A condition that causes the DTC P0300 Engine Misfire Detected may also set the current DTC; conversely an extremely rich condition which sets the current DTC can cause a misfire at Idle and the DTC P0300 to set. If the cause of the current DTC cannot be determined and the DTC P0300 is also set, refer to DTC P0300 Engine Misfire Detected. P0300

Test Description
The numbers below refer to the step numbers on the diagnostic table.

2. Visually and physically checking items which may cause a rich condition may determine the cause of the DTC P0172 being set.

3. This step checks the fuel system for problems.

5. A silicon contamination of the Heated Oxygen Sensor (HO2S Bank 1, Sensor 1) can cause a false rich indication to be seen by the VCM. The VCM then reduces the amount of fuel delivered, possibly causing severe d riveability symptoms.