For 1990-2009 cars

### Description

The powertrain control module (PCM) uses a Closed Loop system in order to meter fuel. This provides the best possible combination of driveability, of fuel economy, and of emission control. The PCM monitors the signal voltage of the heated oxygen sensor (HO2S). When the system is in Closed-Loop operation, the PCM adjusts fuel delivery based on the HO2S signal voltage. The scan tool will display changes in the fuel delivery by indicating the values of the long term and of the short term fuel trim. The best fuel trim value is around 0 percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Fuel trim values that are above 0 percent indicate additional fuel. The PCM will reduce the amount of fuel that is delivered when the HO2S indicates a rich condition. Fuel trim values below 0 percent indicate a reduction in fuel. A fuel trim DTC may set when exhaust emissions reach excessive levels because of a lean condition or because of a rich condition. Conditions for Running the DTC

### Conditions for Running the DTC

• | Barometric pressure is greater than 75 kPa.

| The fuel tank level is more than 25 percent.

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

• | The engine is running at a steady speed.

• | The engine is operating in Closed-Loop.

### Conditions for Setting the DTC

• | The total fuel trim is more than 30 percent or equal to 30 percent for 5 seconds.

• | The short term fuel trim is more than 20 percent or equal to 20 percent for 45 seconds.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles during 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 exhaust gas recirculation (EGR) control and evaporative emissions (EVAP) purge control.

### Conditions for Clearing the MIL/DTC

• | The MIL turns OFF after three consecutive trips during which the diagnostic runs and the fault is not present.

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

• | Use the scan tool Clear 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, it may be necessary to monitor the fuel pressure while driving the vehicle at various road speeds. 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. Too much alcohol in the fuel may also cause a lean exhaust indication. Refer to Alcohol/Contaminants-in-Fuel Diagnosis

• | A MAP sensor output that causes the PCM to sense a lower manifold pressure may cause the system to go lean. By disconnecting the MAP sensor, the PCM may substitute a fixed value for the MAP sensor. If the lean condition disappears when you disconnect the MAP sensor, substitute a known good MAP sensor, and check the lean condition again.

• | 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 the 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. Before replacing the HO2S, determine the cause of the contamination, and correct the condition.

• | Inspect the HO2S 1 for water intrusion into the wiring harness and into the sensor housing. Water can create a short to ground in the HO2S signal circuit that causes 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 Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was set.