The short term fuel trim is used by the control module in order to control the short term injector pulse width, or the injector ON time. When the control module is in the Closed Loop mode, the control module monitors the fuel trim oxygen sensors signals in order to determine if the fuel system is operating rich or lean. If the control module determines that no change in fuel delivery is needed in order to maintain correct short term fuel control for a particular engine operating condition, a short term fuel trim scan tool data value of 0 percent (128 counts) is indicated. If the control module determines that the fuel system is operating rich, short term fuel trim will compensate for the rich condition by decreasing the injector pulse width and a short term fuel trim value of less than 0 percent (128 counts) is indicated. If the control module determines that the fuel system is operating lean, short term fuel trim will compensate for the lean condition by increasing the injector pulse width and a short term fuel trim value of more than 0 percent (128 counts) is indicated.

When the control module is in the power enrichment (PE) mode, the short term fuel trim value will indicate 0 percent (128 counts) in order to prevent the fuel trim from compensating for the commanded rich condition of the PE mode.