For 1990-2009 cars

### 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 heated oxygen sensor (HO2S) input. During Closed Loop the PCM adds HO2S inputs and level of purge to calculate short term and long term fuel trim adjustments. If the HO2S indicate a lean condition, fuel trim values will be above 0 percent. If the HO2S 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 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.

### DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0171 Fuel Trim System Lean

### Conditions for Running the DTC

• | DTCs P0030, P0036, P0068, P0101, P0102, P0103, P0106, P0107, P0108, P0116, P0117, P0118, P0120, P0121, P0122, P0123, P0128, P0130-P0138, P0140, P0141, P0201-P0204, P0220, P0222, P0223, P0300, P0442, P0443, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0496, P0506, P0507, P060D, P060E, P1101, P1106, P1107, P1114, P1115, P1133, P1134, P1380, P1381, P1516, P2101, P2119, P2120, P2122, P2123, P2125, P2127, P2128, P2135, P2138, P2176 are not set.

• | The engine is in Closed Loop status.

• | The engine coolant temperature (ECT) is between -7 and +120°C (+19.4 and +248°F).

• | The intake air temperature (IAT) is between -7 and +145°C (+19.4 and +293°F).

• | The manifold absolute pressure (MAP) is between 15-100 kPa (2.2-14.5 psi).

• | The vehicle speed is less than 132 km/h (82 mph).

• | The engine speed is between 400-6,100 RPM.

• | The barometric pressure (BARO) is more than 72 kPa (10.4 psi).

• | The mass air flow (MAF) is between 1-512 g/s.

• | The fuel level is more than 10 percent.

• | DTC P0171 runs continuously when the above conditions are met.

### Conditions for Setting the DTC

• | The long term fuel trim weighted average value is more than a calibrated value.

• | 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.

• | 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 .

• | 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, a 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 Testing for Intermittent Conditions and Poor Connections .

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | 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.

Are any DTCs other than DTC P0171 also set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3 |

1. Turn the ignition ON.
2. Observe the Freeze Frame/Failure Records parameter with a scan tool and record the displayed data for this DTC.
3. Start and operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions observed from the Freeze Frame/Failure Records.
4. Observe the Long Term FT parameter.

Does the scan tool indicate that the long term fuel trim is more than the specified value?

19%

Go to Step 4

Go to Diagnostic Aids

4 | Slowly operate the engine from idle to 3,000 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 |

1. Turn OFF the engine.
2. Visually and physically inspect the following items:

• | The vacuum hoses for splits, kinks, and proper connections--Refer to Emission Hose Routing Diagram .

• | Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Did you find and correct the condition?

--

Go to Step 9

Go to Step 8

6 |

1. Turn OFF the engine.
2. Inspect the HO2S for proper installation. Refer to Heated Oxygen Sensor 1 Replacement and Heated Oxygen Sensor 2 Replacement .
3. Ensure that the electrical connectors and wires are secured and not contacting the exhaust system.

Did you find and correct the condition?

--

Go to Step 9

Go to Step 7

7 | Test the HO2S high signal circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | -- | Go to Step 9 | Go to Fuel System Diagnosis
8 |

1. Operate the engine at idle.
2. Inspect for the following conditions:

• | Missing, loose, or leaking exhaust components, from the HO2S forward--Refer to Symptoms - Engine Exhaust .

• | Vacuum leaks at the intake manifold, the throttle body, and the injector O-rings

• | Air induction system and the air intake ducts for leaks or for a missing air filter element--Refer to Air Cleaner Assembly Replacement and Air Cleaner Resonator and Outlet Duct Replacement .

• | Evaporative canister purge lines plugged or obstructed

• | Properly functioning fuel injectors--Refer to Fuel Injector Diagnosis .

• | Crankcase ventilation system for leaks--Refer to Positive Crankcase Ventilation Hose/Pipe/Tube Replacement .

Did you find and correct the condition?

--

Go to Step 9

Go to Symptoms - Engine Mechanical

9 | Important: After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim.

1. Turn ON the ignition, with the engine OFF.

Important: DO NOT clear codes with the engine running. The codes may reset in the same ignition cycle.

2. Clear the DTCs with a scan tool.
3. Turn OFF the ignition for 30 seconds.
4. Start the engine.
5. Operate the vehicle within the Conditions for Running in the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 10

10 | To verify that the performance of the catalytic converter has not been affected by the condition that set this DTC, perform the Repair Verification for DTC P0420. Refer to DTC P0420 . Did you complete the procedure? | -- | Go to Step 11 | Go to DTC P0420
11 | 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 | System OK