The long term fuel trim consists of groups of fuel control calibrations or cells. Each cell contains the short term and long term fuel injector control values required to maintain the correct air/fuel mixture for a particular long term engine operating condition. The powertrain control module (PCM) monitors the short term fuel trim values in order to determine if the short term fuel trim values are indicating a trend of rich or lean corrections in the fuel control. If the PCM determines that the short term fuel trim is able to maintain control and is near 0 percent, the long term fuel trim within that particular cell will indicate a value of 0 percent. If the PCM determines that the fuel system is operating rich, long term fuel trim will compensate for the rich condition by decreasing the fuel injector pulse width. When this happens a long term fuel trim value of less than 0 percent is indicated on a scan tool. If the control module determines that the fuel system is operating lean, long term fuel trim will compensate for the lean condition by increasing the injector pulse width. In this case the scan tool indicates a long term fuel trim value of more than 0 percent.

If the PCM determines that the long term fuel trim values indicate a trend toward a rich or a lean correction within that cell, the PCM will switch to the appropriate richer or leaner cell, and the short term fuel trim value will return to 0 percent.