For 1990-2009 cars

The PCM can determine if the engine is running rich when the vehicle operator demands heavy acceleration. When heavy acceleration is demanded, the PCM will go into power enrichment mode. During this mode, a lower air/fuel ratio than 14.7:1 (around 12.0 to 1) will be commanded and the fuel injector pulse width on-time will increase. During this condition, the PCM should detect the heated oxygen sensor 1 (HO2S-1) signal voltage go high and remain there. DTC P1171 sets when the PCM is in power enrichment mode and the HO2S-1 signal voltage is too low, which is considered lean.

### DTC Parameters

DTC P1171 will set if the HO2S-1 voltage is less than 300 mV in power enrichment mode when:

• | The condition exists for longer than 5 seconds.

• | The engine run time is greater than 20 seconds.

• | The ECT is greater than 20°C (68°F).

• | The loop status is closed.

DTC P1171 diagnostic runs continuously once the above conditions have been met.

DTC P1171 is a type D DTC.

### Diagnostic Aids

If DTC P0137 is also set, diagnose that DTC first. A short to ground in the HO2S-1 signal voltage may also set DTC P1171.

DTC P1171 indicates the engine is running lean in power enrichment mode. To verify the system is running lean in power enrichment, use the scan tool to monitor the HO2S-1 and HO2S-2 under heavy acceleration. If both sensors are below 300 mV, the system is running lean and the oxygen sensors are probably not at fault.

The following are possible causes of a lean air/fuel mixture:

• | Fuel pressure low/restricted fuel filter

• | Exhaust system restricted

• | Vacuum leak

• | Fuel quality poor or water/alcohol in fuel

• | Exhaust manifold gasket leak or leak around HO2S-1

• | Fuel injectors clogged

• | HO2S-1 weak