P0171

DTC P0171

Circuit Description

The engine control module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. Fuel delivery is controlled differently during Open and Closed Loop. During Open Loop, the ECM determines fuel delivery based on sensor signals without oxygen sensor (O2S) input. During Closed Loop, the O2S inputs are added and used by the ECM to calculate short and long term fuel trim fuel delivery adjustments. If the O2S indicate a lean condition, fuel trim values will be above 0 percent. If the O2S indicate a rich condition, fuel trim values will be below 0 percent. Short term fuel trim values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. Long term fuel trim makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. If the ECM detects an excessively lean condition, DTC P0171 or P0174 sets.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

^ DTC P0171 Fuel Trim System Lean Bank 1
^ DTC P0174 Fuel Trim System Lean Bank 2

Conditions for Running the DTC

^ DTCs P0030-P0032, P0036, P0038, P0050-P0052, P0056-P0058, P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0131-P0135, P0137-P0141, P0151-P0155, P0157-P0161, P0221, P0222, P0223, P0300, P0301-P0308, P0335, P0335, P0340, P0341, P0345, P0346, P0365, P0366, P0390, P0391, P0443, P0446, P0458, P0459, P1111, P1112, P1114, P1115, P1137, P1138, P1157, P1158, P1380, P1381, P2135, P2231, P2234 are not set.
^ Closed Loop is active.
^ The engine coolant temperature (ECT) is more than 70�C (158�F).
^ The intake air temperature (IAT) is more than -7�C (+19.4�F).
^ The vehicle speed is less than 137 km/h (85 mph).
^ The engine speed is more than 500 RPM.
^ The barometric pressure (BARO) is more than 70 kPa (10.1 psi).
^ The fuel level is more than 9.8 percent.
^ DTC P0171 and P0174 run continuously once the above conditions have been met.

Conditions for Setting the DTC

^ The combined value of the Short Term FT and the Long Term FT parameters are more than 25 percent.
^ The above condition is present for more than 100 seconds.

Action Taken When the DTC Sets

^ The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

^ The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

^ Excessive resistance in the fuel injector ignition 1 voltage circuit or the fuel injector control circuit may cause the following symptoms:

- Lean condition
- Misfire
- Rough idle

^ The system will go lean if an injector is not suppling enough fuel.
^ A lean condition could be present during high fuel demand.
^ A malfunctioning mass air flow sensor can cause this DTC to set. Refer to DTC P0101.
^ Review the Failure Records with a scan. If an intermittent condition is suspected, refer to Intermittent Conditions.