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. This DTC is continuously monitored during engine operation. 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 P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0125, P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0141, P0201-P0204, P0300, P0301-P0304, P0326, P0336, P0340, P0341, P0442, P0446, P0452, P0453, P0455, P0481, P0496, P0502, P0503, P0506, P0601, P0602, P0604, P0606, P1133, and P1621 are not set. |
| • | The engine is in Closed Loop status. |
| • | The engine coolant temperature (ECT) is between 60-115°C (140-239°F). |
| • | The intake air temperature (IAT) is between -25°C to +115°C (-13°F to +239°F). |
| • | The manifold absolute pressure (MAP) is more than 26 kPa (3.8 psi). |
| • | The vehicle speed is less than 132 km/h (82 mph). |
| • | The engine speed is between 550-3,600 RPM. |
| • | The barometric pressure (BARO) is more than 72 kPa (10.4 psi). |
| • | DTC P0171 runs continuously once the above conditions are met. |
Conditions for Setting the DTC
| • | The average long term fuel trim value is above 20 percent. |
| • | The above condition is present for 16 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 pump enough fuel, a plugged fuel filter, or a restricted fuel pipe. |
| • | Fuel contamination, such as water and alcohol, will affect the fuel trim. |
| • | Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions . |
| • | 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 . |
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||||||
2 | Are any DTCs other than P0171 also set? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 3 | ||||||||
3 |
Is the Long Term FT parameter more than the specified value? | 20% | Go to Step 4 | Go to Diagnostic Aids | ||||||||
4 |
Does the scan tool indicate that the HO2S parameters 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 |
Do the MAP parameters 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 | ||||||||
9 |
Important: After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim.
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? | -- | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | System OK |
