Short Term Fuel Trim

The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes "fine tune" the engine fueling. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the PCM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the PCM is reducing the amount of fuel in order to compensate for a rich condition.

When the PCM determines that the short term fuel trim is out of the operating range, the following DTCs will set:

• DTC P0171 Bank 1 Too Lean
• DTC P0172 Bank 1 Too Rich