Circuit Description
The engine control module (ECM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The long term fuel trim values change in response to trends in the short term fuel trim. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition. The fuel trim cells are based on the engine speed and load. If the ECM detects an excessively lean condition, the ECM sets DTC P0171.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0171 Fuel Trim System Lean
Conditions for Running the DTC
| • | DTCs P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0131, P0132, P0133, P0134, P0135, P0201 - P0204, P0300, P0336, P0337, P0341, P0342, P0351, P0352, P0402, P0404, P0405, P0406, P0443, P1404, P0506, P0507, P1167, and P1171 are not set. |
| • | The engine coolant temperature (ECT) is between 70°C (158°F) and 115°C (239°F). |
| • | The barometric pressure is more than 72 kPa. |
| • | The calculated air flow is between 1.5 g/s and 45 g/s. |
| • | The manifold absolute pressure (MAP) sensor is between 25 kPa and 99.7 kPa. |
| • | The intake air temperature (IAT) is between -40°C (-40°F) and 120°C (248°F). |
| • | The engine speed is between 700 RPM and 6,000 RPM. |
| • | The throttle position (TP) sensor angle is less than 95 percent. |
| • | The vehicle speed is less than 140 km/h (87 mph). |
| • | The fuel system is operating in Closed Loop. |
| • | The ignition 1 voltage is more than 11 volts. |
| • | DTC P0171 runs continuously once the above conditions are met. |
Conditions for Setting the DTC
The average long term fuel trim cell values are more than 30 percent for 10 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
| • | The system will become lean if a fuel injector is not supplying enough fuel. |
| • | A lean condition could be present during high fuel demand due to a fuel pump that does not deliver enough fuel, a plugged fuel filter, or a restricted fuel pipe. |
| • | Test for fuel contamination. Water or alcohol can effect fuel trim. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . |
| • | Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions . |
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views | ||||||||||||
1 | Did you perform the Diagnostic System Check - Engine Controls? | -- | Go to Step 2 | |||||||||
2 | Are any DTCs other than P0171 also set? | -- | Go to Step 3 | |||||||||
3 |
Is the Long Term FT more than the specified value? | 30 % | Go to Step 4 | Go to Diagnostic Aids | ||||||||
4 |
Is the voltage within the specified range and fluctuating? | 200-800 mV | Go to Step 5 | Go to Step 6 | ||||||||
5 |
Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 8 | ||||||||
6 |
Does the MAP parameter indicate the correct barometric pressure? | -- | Go to Step 7 | Go to DTC P0106 | ||||||||
7 |
Did you find and correct the condition? | -- | Go to Step 9 | Go to Fuel System Diagnosis | ||||||||
8 |
Did you find and correct the condition? | -- | Go to Step 9 | Go to Symptoms - Engine Mechanical in Engine Mechanical - 2.0L (L34GMDAT) | ||||||||
9 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 10 | ||||||||
10 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |||||||||
