Short Term Fuel Trim

SHORT TERM FUEL TRIM
The short term fuel trim is a Powertrain Control Module (PCM) erasable memory register that will contain a number between 0 and 255. The neutral value for the short term fuel trim is 128, any deviation from this value indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 128. The short term fuel trim changes the pulse width by varying the "Closed Loop" factor of the base pulse width equation. As the PCM monitors the oxygen sensor's input, it is constantly varying the short term fuel trim value.

The value is updated very quickly, therefore, the short term fuel trim only corrects for short term mixture trends. The correction of long term mixture trends is the function of long term fuel trim.

When the PCM determines that the Short Term Fuel Trim is out of the operating range, the following DTC's will set:
- DTC P0171 Bank 1 Too Lean
- DTC P0172 Bank 1 Too Rich
- DTC P0174 Bank 2 Too Lean
- DTC P0175 Bank 2 Too Rich