Fuel Trim Cell Diagnostic Weights

No fuel trim DTC will set regardless of the fuel trim counts in cell 0 unless the fuel trim counts in the weighted cells are also outside specifications. This means that the vehicle could have a fuel trim problem that is causing a driveability concern but the DTC does not set. For example the engine idles high due to a small vacuum leak or the engine idles rough due to a large vacuum leak. In either example the engine operates fine at any other time, and no fuel trim DTC would set. Yet ECM could set an engine idle speed DTC or heated oxygen sensor (HO2S) 2 DTC because of the decline in engine performance. Use a scan tool to observe fuel trim counts when the problem is occurring.