P0171

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 lean condition is detected, the VCM sets this 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

^ No Misfire DTCs

^ Throttle position is less than 69.9%

^ Engine speed is greater than 525 RPM but less than 4500 RPM

^ The BARO is greater than 70 kPa

^ The ECT is 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 is greater than -20°C but less than 69.7°C

^ The IAT is greater than -20°C but less than 80°C

^ MAF is greater than 3 grams per second but less than 150 grams per second

^ The vehicle speed is 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 MIL (Malfunction Indicator Lamp). The MIL remains on during all of 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 drive trips when the test has Run and Passed and not Failed.

^ A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during the same ignition cycle).

^ Use the scan tool Clear Information function.

^ Disconnect the VCM battery feed for 30 seconds.

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 lean 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. On vehicles equipped with a 7.4L (California emissions), an AIR Pump that is stuck ON may also cause a DTC P0171, or DTC P0174 to set. P0300

TEST DESCRIPTION

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

2. Visually and physically check the items that can cause a lean condition in order to determine the cause of the DTC being set.

6. This step checks for a fault in the EVAP purge system which can cause DTC P0171 to be set.