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 that is based on sensor signals, without oxygen sensor input. During Closed Loop the PCM adds oxygen sensor inputs to calculate short and long term fuel trim fuel delivery adjustments. If the oxygen sensors 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. If the PCM detects an excessively lean condition, DTC P0171 will set.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0171 Fuel Trim System Lean Bank 1
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0107, P0108, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0201, P0202, P0203, P0204, P0205, P0206, P0300, P0401, P0403, P0404, P0405, P0442, P0443, P0446, P0449, P0506, P0507, or P1404 are not set. |
| • | The engine coolant temperature (ECT) is between 20-110°C (68-230°F). |
| • | The intake air temperature (IAT) is between 18-70°C (64-158°F). |
| • | The manifold absolute pressure (MAP) is between 15-105 kPa (2.1-15.2 psi). |
| • | The vehicle speed is less than 132 km/h (82 mph). |
| • | The engine speed is between 600-4,000 RPM. |
| • | The barometric pressure (BARO) is more than 70 kPa (10.1 psi). |
| • | The idle purge is enabled. |
| • | The mass air flow (MAF) is between 5-150 g/s. |
| • | The fuel level is more than 10 percent. |
| • | The scan tool is not commanding device control. |
Conditions for Setting the DTC
| • | The value average of the long term fuel trim is above 20 percent. |
| • | All of the above conditions are present for 6 seconds. |
| • | This diagnostic runs continuously when the above conditions have been met. |
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 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 that does not pump enough fuel. |
| • | Excessive resistance in the ignition 1 voltage and/or fuel injector circuits may cause the following symptoms: |
| - | A lean condition |
| - | Misfire |
| - | Rough idle |
| • | Fuel contamination, such as water and alcohol, will affect fuel trim. |
| • | Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions . |
| • | This DTC may set due to poor idle characteristics. A possible cause may be uncontrolled fueling due to an open or high resistance in the HO2S 1 low signal circuit. Before replacing any component, ensure that this condition does not exist. |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
Refer to Fuel System Diagnostics for a possible fuel condition if conditions were not corrected.
-
If conditions were not corrected, a worn cam, worn intake, exhaust values, or other engine mechanical failures may be at fault.
Step | Action | Values | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||||||||
2 |
Important: Refer to the DTCs that are set, other than P0171, before continuing.
Does the scan tool indicate that the long term fuel trim is greater than the specified value? | 23% | Go to Step 3 | Go to Diagnostic Aids | ||||||||||
3 |
Does the scan tool indicate that the values are within the specified range and fluctuating? | 200-800 mV | Go to Step 4 | Go to Step 5 | ||||||||||
4 |
Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 8 | ||||||||||
Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 6 | |||||||||||
6 | Test the HO2S 1 low signal circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 9 | Go to Step 7 | ||||||||||
7 | Test for a short between the signal circuit and the low reference circuit or ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 9 | Go to Fuel System Diagnosis | ||||||||||
Did you find and correct the condition? | -- | Go to Step 9 | ||||||||||||
9 |
Important: After repairs, use the Fuel Trim Reset function on the scan tool to reset the long term fuel.
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 |
