Short Term Fuel Trim

The short term fuel trim (Integrator) is a control module volatile memory register that contains 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.