The Long Term Fuel Trim is a matrix of cells arranged by RPM and MAP. Each cell of the Long Term Fuel Trim is a register like the Short Term Fuel Trim. As the engine operating conditions change, the PCM will switch from cell to cell to determine what Long Term Fuel Trim factor to use in the base pulse width equation.

While in any given cell, the PCM also monitors the Short Term Fuel Trim. If the Short Term Fuel Trim is far enough from 0 percent, the PCM will change the Long Term Fuel Trim value. Once the Long Term Fuel Trim value is changed, it should force the Short Term Fuel Trim back toward 0 percent. If the mixture is still not correct (as judged by the HO2S), the Short Term Fuel Trim will continue to have a large deviation from the ideal 0 percent. In this case, the Long Term Fuel Trim value will continue to change until the Short Term Fuel Trim becomes balanced. Both the Short Term Fuel Trim and Long Term Fuel Trim have limits which vary by calibration. If the mixture is off enough so that Long Term Fuel Trim reaches the limit of its control and still cannot correct the condition, the Short Term Fuel Trim would also go to its limit of control in the same direction. If the mixture is still not corrected by both Short Term Fuel Trim and Long Term Fuel Trim at their extreme values, a Fuel Trim Diagnostic Trouble Code (DTC) will likely result.

Under the conditions of power enrichment, the PCM sets the Short Term Fuel Trim to 0 percent and freezes it there until power enrichment is no longer in effect. This is done so the Closed Loop factor and the Long Term Fuel Trim will not try to correct for the commanded richness of power enrichment.