1.1.1 Diagnostic Overview

The catalyst diagnostic is based on monitoring the Oxygen Storage Capability (OSC), which is achieved by comparing the signals from the front O2S and the rear O2S.

The engine control stimulates the regular exhaust gas lambda oscillations, which are needed for best possible catalyst conversion. The catalyst storage activity damps these oscillations and the amplitude of the remaining lambda oscillations downstream of the catalyst indicates the OSC.

The efficiency of the catalyst system is tested during steady state driving by cycling the air fuel ratio (AFR) to lean and then to rich for a calibrated number of cycles while monitoring the OSC.

The EVAP CPV is closed or opened with a low canister purge value prior to the catalyst test. This process eliminates the influence the canister vapors have on the rear O2Ss during the test.