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 (CL). During Open Loop, the ECM determines fuel delivery based on sensor signals, without heated oxygen sensor (HO2S) input. During CL, the ECM adds HO2S inputs to calculate the short and long term fuel trim (FT) adjustments. If the HO2S indicates a lean condition, the fuel trim values will be above 0 percent. If the HO2S indicates a rich condition, the FT values will be below 0 percent. The short term FT values change rapidly in response to the HO2S voltage signals. The long term FT makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. A block of cells contain information arranged in combinations of engine RPM and engine load for a full range of vehicle operating conditions. The long term FT diagnostic is based on an average of cells currently being used. The ECM selects the cells based on the engine speed and load. If the ECM detects an excessively lean condition, DTC P0171 sets.

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

CONDITIONS FOR RUNNING THE DTC
- DTCs P0030, P0036, P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0116, P0117, P0118, P0120, P0121, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0136, P0137, P0138, P0140, P0141, P0201-P0204, P0220, P0222, P0223, P0300, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0496, P0506, P0507, P1106, P1107, P1114, P1115, P1133, P1134, P1258, P1516, P1680, P2101, P2119, P2120, P2122, P2123, P2125, P2127, P2128, P2135, P2138, P2176, P2A01 are not set.
- The engine is in CL status.
- The engine coolant temperature (ECT) is between -7 and +120°C (19.4-248°F).
- The intake air temperature (IAT) is between -7 and +145°C (19.4-293°F).
- The manifold absolute pressure (MAP) is between 15-100 kPa (2.2-14.5 psi).
- The vehicle speed is less than 132 km/h (82 mph).
- The engine speed is between 400-6,100 RPM.
- The mass air flow (MAF) is between 1-512 g/s.
- The barometric pressure (BARO) is more than 72 kPa (10.4 psi).
- The fuel level is more than 10 percent.
- DTC P0171 runs continuously once the above conditions have been met.

CONDITIONS FOR SETTING THE DTC
- The long term FT weighted average value is more than a calibrated value.
- 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) 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
- Test for fuel contamination. Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. A lean exhaust indication can also be caused by too much alcohol in the fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis (without Special Tool) Alcohol/Contaminants-in-Fuel Diagnosis (with Special Tool). Without Special Tool With Special Tool
- The system will go lean 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, or a restricted fuel pipe.
- Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Testing for Intermittent Conditions and Poor Connections. Component Tests and General Diagnostics

TEST DESCRIPTION












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