The Short Term Fuel Trim is a PCM erasable memory register. The neutral value for the Short Term Fuel Trim is 0 percent. Any deviation from 0 percent 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 0 percent. The Short Term Fuel Trim is rich when the scan tool indicates -99 percent. The Short Fuel Trim is lean when the scan tool indicates 99 percent. 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 sensors 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.