Diagnostic overview

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Catalyst monitoring is based on the monitoring of the o xygen s torage c apability (OSC) by comparing the signals of the O2 sensor upstream and downstream the catalyst. The engine control stimulates the regular lambda oscillations of the exhaust gas. These oscillations are needed for best possible catalyst conversion. They are damped by the storage activity of the catalyst. The amplitude of the remaining lambda oscillations downstream the catalyst indicates the oxygen storage capability.

The efficiency of the catalyst system is tested during steady state driving by cycling the air fuel ratio LEAN and then RICH for a calibratable number of cycles while monitoring the OSC.

Prior to the catalyst test the canister purge valve is closed or opened with low canister purge value. This is to eliminate the influence of canister vapors on the downstream sensor during the test.