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 and Closed Loop. During Open Loop the PCM determines fuel delivery based on sensor signals without oxygen sensor input. During Closed Loop the PCM adds oxygen sensor inputs and level of purge to calculate Short and Long Term fuel trim adjustments. If the oxygen sensors (O2S) indicate a lean condition, fuel trim values will be above 0 percent . If the oxygen sensors indicate a rich condition, fuel trim values will be below 0 percent. The values for the Short Term fuel trim 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. 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, but is capable of identifying a lean condition within a single cell. The PCM determines the cell based on the current engine speed and engine load. If the PCM detects an excessively lean condition, DTC P0171 sets.
Conditions for Running the DTC
| • | DTCs P0030, P0036, P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0117, P0118, P0120, P0121, P0125, P0128, P0130-P0138, P0140, P0141, P0201-P0204, P0300, P0420, P0440, P0442, P0443, P0446, P0449, P0452, P0453, P0461, P0462, P0463, P0496, P0506, P0507, P0607, P1221, P1271, P1275, P1280, P1380, P1381, P1441, P1512, P1516 are not set. |
| • | The engine is in Closed Loop status. |
| • | The engine coolant is more than -7°C (19°F). |
| • | The intake air temperature (IAT) is more than -7°C (19°F). |
| • | The manifold absolute pressure (MAP) is between 15-85 kPa (2.1-12.3 psi). |
| • | The vehicle speed is less than 132 km/h (82 mph). |
| • | The engine speed is between 400-5,700 RPM. |
| • | The barometric pressure (BARO) is more than 74 kPa (10.7 psi). |
| • | The mass air flow (MAF) is between 1-511 g/s. |
| • | The fuel level is more than 10 percent. |
Conditions for Setting the DTC
| • | The Long Term fuel trim is above 22 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) 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 current cell can be observed using a scan tool. |
| • | The system will go lean if an injector is not providing enough fuel. |
| • | A lean condition could be present during high fuel demand due to a fuel pump not pumping enough fuel, plugged fuel filter or a restricted fuel line. |
| • | Insufficient fuel in the tank or low fuel pressure may cause this DTC to set. Refer to Fuel System Diagnosis . |
| • | Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions . |
Step | Action | Values | Yes | No | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Engine Controls? | -- | Go to Step 2 | |||||||||||||
2 | Are any DTCs other than DTC P0171 also set? | -- | Go to Step 3 | |||||||||||||
3 |
Does the scan tool indicate that the Long Term fuel trim is more than the specified value? | 22 % | Go to Step 4 | Go to Diagnostic Aids | ||||||||||||
4 | Slowly operate the engine from idle to 3000 RPM while observing the HO2S 1 and HO2S 2 parameters with a scan tool. Does the scan tool indicate that the heated oxygen sensor (HO2S) voltages are 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 |
Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 7 | ||||||||||||
7 | Test the HO2S signal circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. 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 | ||||||||||||
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 |
