System Description
WARNING: This page is about a different car, the 2006 Chevrolet Monte Carlo and 2006 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
The air fuel imbalance monitor (AFIM) diagnostic detects a rich or lean cylinder to cylinder air/fuel ratio imbalance. An imbalance will result in oxygen sensor signal switching above the desired frequency. Diagnostic detection is accomplished by measuring the frequency of the pre-catalyst oxygen sensor signal. If the frequency of the oxygen sensor signal exceeds the calibrated limit, a failure is reported.