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 of their thresholds, a rich or a lean DTC will record.

In order to meet OBD II requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only set if the fuel trim counts in the weighted fuel trim cells exceed specifications. This means that the vehicle could have a fuel trim problem which is causing a concern under certain conditions (for example, engine idle high due to a small vacuum leak or an engine idle rough due to a large vacuum leak) while the engine operates normal at other times. No fuel trim DTC would set (although an engine idle speed DTC or a HO2S DTC may set). Remember, use a scan tool in order to observe the 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 of the information available (other DTCs stored, a rich or a lean condition, etc.) when diagnosing a fuel trim fault.