For 1990-2009 cars

### 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.

### Conditions for Running the DTC

• | Barometric pressure greater than 75 kPa.

• | Fuel tank level greater than 25%.

• | Intake air temperature is between -10°C to 70°C (14°F to 158°F).

• | Engine is running at a steady speed.

• | Engine is operating in Close Loop.

### Conditions for Setting the DTC

• | Total fuel trim is greater than or equal to 30% for 5 seconds.

• | Short term fuel trim is greater than or equal to 20% for 45 seconds.

### 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.

• | The PCM enters the Fail-Safe Function and stops EGR control and EVAP purge control.

### 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 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.

• | 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.

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 Powertrain 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. This step determines that the fault is present.

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. This step checks for inadequate fuel delivery from the fuel injector. The fault may be the fuel injector or the fuel injector electrical circuit.

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Start the engine.
2. Attain normal operating temperature.
3. Operate the vehicle while a scan tool 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. Perform the scan tool Clear DTC Information function.
3. Operate the vehicle within the Freeze Frame conditions as specified.
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?

30%

Go to Diagnostic Aids

Go to Step 4

4 |

1. Visually and physically check for any of the following conditions:

• | Vacuum hoses for splits, kinks and improper connections.

• | PCV valve for proper operation.

• | Exhaust system for leaks.

• | EGR valve for proper operation.

2. Repair as necessary.

Did 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 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 11

Go to Step 6

6 |

1. Turn OFF the ignition.
2. Install the J 34730-1A fuel pressure gauge or an equivalent. Refer to Fuel Pressure Gage Installation and Removal .
3. Start the engine.
4. Observe the fuel pressure with the engine idling at normal operating temperatures.

Is the fuel pressure within the specified value?

90-140 kPa (13-20 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. Refer to Fuel System Cleaning . Is the action complete? | -- | Go to Step 11 | --
9 | Check the fuel injector for any of the following conditions:

• | The discharge of fuel is regular and even.

• | The fuel is dispersed in a conical spray pattern.

• | The electrical resistance of the injector coil is within the specified value.

Did the fuel injector pass all the checks?

0.5 to 1.5 ohms at 20°C (68°F)

Go to Diagnostic Aids

Go to Step 10

10 | Repair/Replace the fuel injector. Refer to Fuel Injector Replacement . Is the action complete? | -- | Go to Step 11 | --
11 |

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