Long Term Fuel Trim

The long-term Fuel Trim (FT) is a matrix of cells arranged by the RPM and the MAP. Each cell of the long-term FT is a register like the short-term FT. As the engine operating conditions change, the PCM will switch from cell to cell in order to determine what long-term FT factor to use in the base pulse width equation.

While in any given cell, the PCM also monitors the short-term FT. If the short-term FT is far enough from 0 percent, the PCM will change the long-term FT value. Once the long-term FT value is changed, the change should force the short-term FT back toward 0 percent. If the mixture is still not correct, as judged by the PCM, the short-term FT will continue to have a large deviation from the ideal 0 percent. In this case, the long-term FT value will continue to change until the short-term FT becomes balanced. Both the short-term FT and long-term FT have limits which vary by calibration. If the mixture is off enough so that long-term FT reaches the limit of its control and still cannot correct the condition, the short-term FT would also go to its limit of control in the same direction. If the mixture is still not corrected by both short-term FT and long-term FT at their extreme values, a FT DTC will likely result. When the PCM determines that the long-term FT is out of the operating range, the following DTCs will set:
^ DTC P0171 FT System Lean Bank 1
^ DTC P0172 FT System Rich Bank 1
^ DTC P0174 FT System Lean Bank 2
^ DTC P0175 FT System Rich Bank 2

Under the conditions of power enrichment, the PCM sets the short-term FT to 0 percent and freezes the FT there until the power enrichment is no longer in effect. This is done so the Closed Loop factor and the long-term FT will not try to correct for the commanded richness of power enrichment.