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 Loop and Closed Loop. During Open Loop, the ECM determines fuel delivery based on sensor signals, without oxygen sensor (O2S) input. During Closed Loop, the ECM adds oxygen sensor inputs to calculate the short and long term fuel trim. If the oxygen sensors indicate a lean condition, the fuel trim values will be above 0 percent. If the oxygen sensors indicate a rich condition, the fuel trim values will be below 0 percent. The short term fuel trim values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. The long term fuel trim (FT) 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 sets.

DTC DESCRIPTOR
This diagnostic supports the following DTC:
DTC P0171 Fuel Trim System Lean

CONDITIONS FOR RUNNING THE DTC
- DTCs P0037, P0038, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0122, P0123, P0125, P0130, P0131, P0132, P0133, P0134, P0137, P0138, P0140, P0141, P0300, P0336, P0340, P0341, P0401, P0404, P0455, P0500, P0506, P0507, P0601, P0602, P0606, P0641, P1133, P1134 are not set.
- The engine coolant temperature (ECT) is between 60-115°C (140-239°F).
- The intake air temperature (IAT) is between -20 to +80°C (-4 to +176°F).
- The manifold absolute pressure (MAP) is between 30-90 kPa (4.3-13 psi).
- The engine speed is between 500-4,000 RPM.
- The barometric pressure (BARO) is more than 70 kPa (10.1 psi).
- The throttle angle is less than 75 percent.
- DTC P0171 runs continuously once the above conditions have been met.

CONDITIONS FOR SETTING THE DTC
- The average long term FT value is more than 20 percent.
- The above condition is present for more than 2 minutes.

ACTION TAKEN WHEN THE DTC SETS
- The control module illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
- The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Freeze Frame/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
- A lean condition can result if an injector is not supplying enough fuel.
- A lean condition could be present during high fuel demand due to a fuel pump that does not pump enough fuel.
- Using a scan tool, review the Failure Records. If an intermittent condition is suspected, refer to Intermittent Conditions. Diagnostic Strategies

TEST DESCRIPTION

Steps 1 - 6:

Steps 7 - 9:

The number below refers to the step number on the diagnostic table.
7. If conditions were not corrected, a worn cam, worn intake or exhaust valves, or other engine mechanical failures may be at fault.