Fuel Trim System Monitor Diagnostic Operation

The fuel system monitor diagnostic 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 the short-term fuel trim values and the long-term fuel trim values to the 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 DTC will be recorded.

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 be set if the fuel trim counts in the weighted fuel trim cells exceed the specifications. A vehicle that has a fuel trim problem that is causing a concern under certain conditions but operates fine under other conditions may not set a fuel trim DTC. For example an engine that is idling high due to a small vacuum leak or an engine that is running rough due to a large vacuum leak may set an idle speed DTC or an HO2S DTC but not a fuel trim DTC.

A fuel trim DTC may be triggered by many different vehicle faults. Use all diagnostic information available when diagnosing a fuel trim fault.