For 1990-2009 cars

### Circuit Description

The DTC P0171 Fuel Trim System Lean diagnostic monitors the heated oxygen sensor indications of the air/fuel mixture. In order to provide the best possible combination of driveability, fuel economy, and emission control, the powertrain control module (PCM) uses a closed loop air/fuel metering system. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and when in closed loop adjusts fuel delivery based on the HO2S signal voltage. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around 0 percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above 0 percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below 0 percent indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition.

### Conditions for Running the DTC

• | Barometric pressure is more than 75 kPa.

• | Engine coolant temperature is between -8°C and 95°C (18°F and 203°F).

• | Intake air temperature is between -8°C and 60°C (18°F and 140°F).

### Conditions for Setting the DTC

• | Total fuel trim is more than or equal to 43 percent in 128 firing events.

• | Short term fuel trim is more than or equal to 20 percent in 128 firing events.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The MIL turns OFF after three consecutively passing trips without a fault present.

• | A History DTC clears after 40 consecutive warm-up cycles without a fault.

• | Use the scan tool Clear DTC Information function or disconnect the PCM battery feed in order to clear the DTC.

### Diagnostic Aids

Check for any of the following conditions:

• | Check for vacuum leaks.

• | Check for exhaust leaks.

• | Check for the correct fuel pressure. The fuel system will be lean if fuel pressure is too low. In order to determine the cause of a DTC P0171, monitoring the fuel pressure while driving the vehicle at various road speeds may be necessary. Refer to Fuel System Diagnosis .

• | Check 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 .

• | A malfunctioning MAF sensor.

• | Check for a damaged wiring harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving the connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

• | Check for a contaminated oxygen sensor. Silicone and other contaminants can cause the oxygen sensor to provide false lean exhaust readings. If contamination is found, determine the cause and correct the condition before replacing the HO2S.

• | Inspect the HO2S 1 for water intrusion into the wiring harness and sensor housing. Water can create a short to ground in the HO2S signal circuit causing a false lean indication.

• | Check the HO2S 1 and the MAF sensor for a faulty electrical connection to the PCM.

If the DTC P0171 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0171 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0171 appears in the Not Ran Since Code Cleared list, the P0171 diagnostic has not yet run. When the DTC P0171 does not appear in the Not Ran Since Code Cleared list, the P0171 diagnostic has run. If the MIL is NOT ILLUMINATED and there is no PENDING DTC Status in DTC Information, the P0171 diagnostic has passed. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the Not Ran Since Code Cleared list. DO NOT FORGET that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for CURRENT or PENDING status, in order to determine the outcome of the diagnostic test involved.

If you cannot duplicate a DTC P0171, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

### Test Description

The numbers below refer to the step numbers in the diagnostic table.

1. The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and to store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. If DTCs other than Fuel Trim P0171, P0172, Misfire P0300 to P0304, and EGR P0400 are set, diagnose those DTCs first.

3. Review the Freeze Frame data to determine when the DTC set. Always record this information.

4. Any one of these sensors could cause this DTC to set. Check the performance of the MAF sensor, ECT sensor, TP sensor, and the CTP switch signal. Repair any faulty conditions.

5. Engine control sensors that are found to be out of range (skewed) or fixed, can cause the engine to run lean. Refer to the applicable sensor DTC table for direction in diagnosing any possible sensor error.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 |

1. Start the engine.
2. Install a scan tool.
3. Operate the vehicle until loop status indicates Closed Loop.
4. Observe the Short Term FT parameter on the scan tool.

Is the scan tool display less than the specified value?

20%

Go to Step 3

Go to Step 4

3 |

1. Turn ON the ignition, leaving the engine OFF.
2. Clear the scan tool information.
3. Operate the vehicle within the Freeze Frame conditions as observed.
4. Observe the Total Fuel Trim parameter on the scan tool.

Was the scan tool display less than the specified value while operating the vehicle within the Freeze Frame conditions?

43%

Go to Diagnostic Aids

Go to Step 4

4 | Visually and physically check for the following items:

• | Vacuum hoses for splits, kinks and improper connections

• | The positive crankcase ventilation (PCV) valve for proper installation

• | The exhaust system for leaks

• | The EGR for proper operation

Did any of the above checks isolate a condition requiring a repair?

--

Go to Step 11

Go to Step 5

5 |

1. Start the engine.
2. Run the engine at normal operating temperature.
3. Observe the operation of following sensors at idle, with a scan tool:

• | The manifold absolute pressure (MAP) sensor

• | The engine coolant temperature (ECT) sensor

• | The throttle position (TP) sensor

4. Compare the observed sensors reading with the TYPICAL SCAN TOOL DATA VALUES located in the engine scan tool data list. Refer to Scan Tool Data List .
5. Inspect the vehicle for any errant sensor readings and repair as necessary.

Was a repair necessary?

--

Go to Step 11

Go to Step 6

6 |

1. Turn OFF the ignition.
2. Install a fuel pressure gauge. Refer to Fuel Pressure Gage Installation and Removal .
3. Start the engine.
4. Observe the fuel pressure with the engine idling at normal operating temperature.

Is the fuel pressure within the specified value?

210-260 kPa (30-37 psi)

Go to Step 7

Go to Fuel System Diagnosis

7 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was the fuel contaminated? | -- | Go to Step 8 | Go to Step 9
8 | Replace any contaminated fuel. Is the action complete? | -- | Go to Step 11 | --
9 |

1. Check fuel injectors for clogging or any other restrictions.
2. Replace as necessary.

Was a replacement necessary?

--

Go to Step 11

Go to Step 10

10 | Important: Before replacing the oxygen sensor because of contamination, determine and correct the cause of the contamination.

1. Remove the HO2S 1.
2. Visually inspect the HO2S 1 for damage or contamination.
3. Replace the HO2S 1 if any damage or contamination is present. Refer to Heated Oxygen Sensor 1 Replacement .

Did the HO2S 1 require replacement?

--

Go to Step 11

Go to Symptoms - Engine Controls

11 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 12

Go to Step 2

12 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK