For 1990-2009 cars

### Circuit Description

The DTC P0171 Fuel Trim System Lean diagnostic monitors the heated oxygen sensor (HO2S) 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.

### Condition for Running the DTC

• | The engine coolant temperature (ECT) is more than 70°C (158°F).

• | The engine is running at steady speed.

• | The engine is operating in Closed Loop.

### Conditions for Setting the DTC

The fuel trim value is constantly high during stable engine operation.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM stores the conditions which were present when the DTC set as Freeze Frame data.

### Conditions for Clearing the MIL/DTC

• | The PCM turns OFF the MIL on the third consecutive trip cycle during which the diagnostic has been run and the fault condition is no longer present.

• | A DTC will clear after 40 consecutive warm-up cycles have occurred without a fault.

• | A DTC can be cleared by using the scan tool Clear DTC Information function.

### 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 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 mass air flow (MAF) sensor output that is incorrect may cause the PCM to command the fuel system to go lean. Check the performance of the MAF sensor. Inspect for air restrictions and air inlet leaks near the MAF sensor. If the HO2S voltage is fixed low, while the fuel trim is constantly high, check the MAF sensor signal circuit for resistance. High circuit resistance can cause the sensor to read a lower volume of air than actual, resulting in a lean fueling condition.

• | 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 of the contamination and correct the condition before replacing the HO2S.

• | Check for fuel contamination of the crankcase. Fuel that has entered the crankcase can release vapors that enter the engine through the positive crankcase ventilation (PCV) system, thereby causing a rich running condition.

• | Inspect the HO2S 1 for water intrusion into the wiring harness and the 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 faulty electrical connections to the PCM.

An intermittent malfunction may be caused by a fault in the HO2S 1 electrical circuit. Inspect the wiring harness and the components for an intermittent condition. Refer to Intermittent Conditions .

Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.

If DTC P0171 cannot be duplicated, 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 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. Visually and physically checking items that may cause a lean condition that may determine the reason that the DTC set.

3. Engine control sensors that are found to be out 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.

4. Contaminants in the fuel, such as alcohol or water, can create a lean condition and set DTC P0171.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
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. Operate the vehicle until the loop status indicates Closed Loop.
3. Observe the Short Term FT parameter display on the scan tool.

Is the Short Term FT parameter displayed on the scan tool less than the specified value?

20%

Go to Diagnostic Aids

Go to Step 3

3 | Visually and physically check for any of the following conditions:

• | Splits, kinks, and improper connections in the vacuum hoses

• | Correct installation of the PCV valve

• | Leaks in the exhaust system

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

--

Go to Step 10

Go to Step 4

4 |

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

• | The mass air flow (MAF) sensor

• | The engine coolant temperature (ECT) sensor

• | The throttle position (TP) sensor

4. Compare the observed sensor readings 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 the cause of any errant sensor readings and repair as necessary.

Was a repair necessary?

--

Go to Step 10

Go to Step 5

5 |

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

Is the fuel pressure within the specified range?

301-347 kPa (44-50 psi)

Go to Step 6

Go to Fuel System Diagnosis

6 | Check the fuel for excessive water, alcohol, or other contaminants. Was the fuel contaminated? | -- | Go to Step 7 | Go to Step 8
7 | Replace the contaminated fuel. Is the action complete? | -- | Go to Step 10 | --
8 |

1. Perform the Fuel Injector Balance Test. Refer to Fuel Injector Balance Test with Special Tool .
2. Repair or replace as necessary.

Was a repair necessary?

--

Go to Step 10

Go to Step 9

9 | Is a lean condition still present? | -- | Go to Symptoms - Engine Controls | Go to Step 10
10 |

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 11

Go to Step 2

11 | 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