For 1990-2009 cars

### Circuit Description

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%. 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%. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below 0% 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

• | Engine coolant temperature is greater than 70°C (158°F).

• | Engine is running at steady speed.

• | Engine is operating in Closed Loop.

### Conditions for Setting the DTC

The fuel trim value is constantly high (positive percent) during stable engine operation.

### 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 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 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 MAP sensor output that causes the PCM to sense a lower than normal manifold pressure (high vacuum) can cause the system to go lean. Disconnecting the MAP sensor will allow the PCM to substitute a fixed (default) value for the MAP sensor. If the lean condition is gone when the MAP sensor is disconnected, substitute a known good MAP sensor and recheck.

• | 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.

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

• | 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 MAP sensor for a faulty electrical connection to the PCM.

An intermittent malfunction may be caused by a fault in the HO2S 1 electrical circuit. Inspect the wiring harness and components for any of the following conditions:

• | Backed out terminals.

• | Improper mating of terminals.

• | Broken electrical connectors locks.

• | Improperly formed or damaged terminals

• | Faulty terminal to wire connections.

• | Physical damage to the wiring harness.

• | A broken wire inside the insulation.

• | Corrosion of electrical connections, splices, or terminals.

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

If a 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 OBD System Check 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/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 | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Start the engine.
2. Operate 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%

Fault Not Present-Go to Diagnostic Aids

Go to Step 3

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

• | Vacuum hoses for splits, kinks and improper connections

• | The PCV valve for proper installation

• | The exhaust system for leaks

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 to normal operating temperatures.
3. Observe the operation of the following sensors at idle, with a scan tool.

• | The MAP sensor

• | The ECT sensor

• | The 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 Engine 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 value?

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 Injector Coil Test/Balance Test Procedure. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) .
2. Repair/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 | Go to Step 10
10 |

1. Perform the scan tool Clear DTC Information function and road test the vehicle within the Freeze Frame conditions that set the DTC.
2. Review the scan tool data and check for DTCs. The repair is complete if no DTCs are stored.

Are any DTCs displayed on the scan tool?

--

Go to the applicable DTC table

System OK