Fuel Trim System Monitor Diagnostic Operation

This system monitors the averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of short-term fuel trim values and long-term fuel trim values to rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If both values are outside their thresholds, a rich or lean Diagnostic Trouble Code (DTC) will be recorded.

The fuel trim system diagnostic also conducts an intrusive test. This test determines if a rich condition is being caused by excessive fuel vapor from the EVAP canister.

Like the misfire diagnostic, fuel trim faults do not necessarily have to occur on two consecutive trips to store a DTC.

In order to meet On Board Diagnostics II (OBD II) requirements, the control module uses weighted fuel trim cells to determine the need to set a fuel trim DTC.
For example, in the image, fuel trim cells 4, 5, and 9 are weighted cells. No fuel trim DTC will set regardless of the fuel trim counts in cell 0. Unless, the fuel trim counts in the weighted fuel trim cells are also outside specifications. This means that the vehicle could have a fuel trim problem which is causing a concern under certain conditions (i.e. engine idle high due to a small vacuum leak or rough due to a large vacuum leak) while it operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or Heated Oxygen Sensor (HO2S) DTC may set). Remember, use a scan tool to observe fuel trim counts while the problem is occurring.

Remember, a fuel trim DTC may be triggered by a list of vehicle faults. Make use of all information available (other DTCs stored, rich or lean condition, etc.) when diagnosing a fuel trim fault.