For 1990-2009 cars

The short term fuel trim (FT) is a PCM erasable memory register. The neutral value for the short term FT is 0 percent. Any deviation from 0 percent indicates the short term FT is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 0 percent. The short term FT is rich when the scan tool indicates a negative number. The short FT is lean when the scan tool indicates a number greater than 0. The short term FT changes the injector pulse width by adding to or subtracting from the base pulse width equation. As the PCM monitors the oxygen sensors input, it is constantly varying the short term FT value. The value is updated very quickly, therefore the short term FT only corrects for short term mixture trends. The correction of long term mixture trends is the function of long term FT.

When the PCM determines that the short term FT is out of the operating range, 1 of the following DTCs will set:

• | DTC P0171 FT System Lean Bank 1

• | DTC P0172 FT System Rich Bank 1

• | DTC P0174 FT System Lean Bank 2

• | DTC P0175 FT System Rich Bank 2