Short Term Fuel Trim

If the heated oxygen sensors (HO2S) are warmed up and the PCM determines the engine can operate near stoichiometric air to fuel ratio (14.7:1 for gasoline), the PCM goes into closed loop fuel control mode. Since an oxygen sensor can only indicate rich or lean, the fuel control strategy must constantly adjust the desired air to fuel ratio rich and lean to get the oxygen sensor to switch around the stoichiometric point. If the times between switches are the same, then the system is actually operating at stoichiometry. The desired air to fuel control parameter is called short term fuel trim (SHRTFT1) where stoichiometry is represented by 0%. Richer (more fuel) is represented by a positive number and leaner (less fuel) is represented by a negative number. Normal operating range for short term fuel trim is +/- 25%. Sometimes the calibration can run the system slightly lean or rich of stoichiometry. This practice is referred to as using bias. For example, the fuel system can be biased slightly rich during closed loop fuel to help reduce NOx.

Values for SHRTFT1 may change a great deal on a scan tool when the engine is operated at different RPM and load points. This is because SHRTFT1 reacts to fuel delivery variability that can change as a function of engine RPM and load. Short term fuel trim values are not retained after the engine is turned off.