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 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 optimum air/fuel ratio. A block of memory cells contains 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 engine 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 Bank 1
Conditions for Running the DTC
| • | DTCs P0030, P0036, P0053, P0054, P0068, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0120, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0201, P0202, P0203, P0204, P0205, P0206, P0220, P0300, P0401, P0403, P0404, P0405, P0406, P0420, P0442, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0462, P0463, P0464, P0496, P0506, P0507, P1133, P1134, P1404, P1516, P2101, P2107, P2108, P2119, P2120, P2125, P2135, P2138, P2A00, P2A01 are not set. |
| • | The engine is in Closed Loop status. |
| • | Fuel Trim Learn is enabled. |
| • | The engine coolant is between -40 and +132°C (-40 and +270°F). |
| • | The intake air temperature (IAT) is between -21 and +150°C (-6 and +302°F). |
| • | The manifold absolute pressure (MAP) is between 5-105 kPa (0.7-15.2 psi). |
| • | The vehicle speed is less than 132 km/h (82 mph). |
| • | The engine speed is between 500-6,000 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 0.5-510 g/s. |
| • | The fuel level is more than 10 percent. |
| • | The scan tool special functions are not active. |
Conditions for Setting the DTC
| • | The average Long Term fuel trim is more than a calibrated value. |
| • | The above condition is present for more than 2 minutes. |
| • | This diagnostic runs continuously. |
Action Taken When the DTC Sets
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 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. The control module illuminates the malfunction indicator lamp (MIL) when one of the following occur:
| • | The control module detects the same fuel trim failure during 2 consecutive trips. |
| • | The control module detects any fuel trim failure during any subsequent trip if the conditions at the time of failure meet the following criteria: |
| - | The engine load is within 20 percent of the previous test that failed. |
| - | The engine speed is within 375 RPM of the previous test that failed. |
| - | The engine coolant temperature is in the same range of the previous test that failed. |
Conditions for Clearing the MIL/DTC
| • | The control module turns OFF the malfunction indicator lamp (MIL) at the beginning of the fourth ignition cycle, after 3 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 and related Freeze Frame data clears after 40 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
| • | A malfunctioning MAF sensor can cause a lean condition and set this DTC. |
| • | A lean condition could be present due to air leaks in the air induction system and related components. |
| • | 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 the fuel injector circuits may cause the following symptoms: |
| - | A lean condition |
| - | Misfire |
| - | Rough idle |
| • | Fuel contamination, such as water and alcohol, will effect the fuel trim. |
| • | Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Testing for Intermittent Conditions and Poor Connections . |
| • | This DTC may set due to poor idle characteristics as a result of uncontrolled fueling caused by 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 number below refers to the step number on the diagnostic table.
Step | Action | Values | Yes | No | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||||||||||
2 | Are any DTCs other than DTC P0171 also set? | -- | Go to Step 3 | |||||||||||||
3 |
Important: Disregard any transmission symptoms, antilock brake system (ABS) indicators, and traction control system (TCS) indicators until any fuel trim faults are repaired. A fuel trim fault may cause default actions such as harsh shifts and illumination of the ABS/TCS indicators.
Does the scan tool indicate that the Long Term fuel trim is more than the specified value? | 18.5% | Go to Step 4 | Go to Diagnostic Aids | ||||||||||||
4 |
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 11 | Go to Step 10 | ||||||||||||
6 |
Does the MAP indicate the correct barometric pressure? | -- | Go to Step 7 | |||||||||||||
7 |
Did you find and correct the condition? | -- | Go to Step 11 | Go to Step 8 | ||||||||||||
8 | 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 11 | Go to Step 9 | ||||||||||||
9 | 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 11 | Go to Fuel System Diagnosis | ||||||||||||
Did you find and correct the condition? | -- | Go to Step 11 | ||||||||||||||
11 |
Important: Reset the Long Term Fuel Trim with the scan tool Fuel Trim Reset function after any of the above repairs.
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 12 | ||||||||||||
12 | In order to ensure that the performance of the catalyst has not been affected by the fault that set this DTC, operate the vehicle within the Conditions for running and setting DTC P0420 or P0430 and verify that DTCs P0420 and P0430 run and pass. Refer to DTC P0420 or DTC P0430 . Does the DTC run and pass? | -- | Go to Step 13 | |||||||||||||
13 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK |
