P0171

DTC P0171

Circuit Description

The powertrain control module (PCM) 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 PCM determines fuel delivery based on sensor signals, without heated oxygen sensor (HO2S) input. During Closed Loop, the PCM adds HO2S inputs and level of purge to calculate the short and long term fuel trim 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 fuel trim values will be below 0 percent. The short term fuel trim values change rapidly in response to the HO2S voltage signals. The long term fuel trim 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 fuel trim diagnostic is based on an average of cells currently being used. The PCM selects the cells based on the engine speed and load. If the PCM 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, P0069, P0101, P0102, P0103, P0106, P0107, P0108, 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, P0301-P0304, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0496, P0506, P0507, P1133, P1134, P1516, P1680, P1681, P2101, P2119, P2120, P2122, P2123, P2125, P2127, P2128, P2135, P2138, P2176 are not set.
* The engine is in Closed Loop status.
* The engine coolant temperature (ECT) is between -7 and +120°C (+19.4 and +248°F).
* The intake air temperature (IAT) is between -7 and +145°C (+19.4 and +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 and 6,100 RPM.
* The mass air flow (MAF) is between 1-512 g/s.
* The barometric pressure (BARO) is greater than 72 kPa (10.4 psi).
* The fuel level is greater 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.
* 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, a plugged fuel filter, or a restricted fuel pipe.
* Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions. Intermittent Conditions

Test Description

Step 1 - Step 7:

Step 8 - Step 11:

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.