1.1.1 Diagnostic Overview

Catalyst monitoring is based on monitoring the Oxygen Storage Capability (OSC). This is done by comparing the signals from the O2 sensor upstream and downstream of the catalyst.

The engine control stimulates the regular lambda oscillations of the exhaust gas, which are needed for best possible catalyst conversion. The storage activity of the catalyst 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 LEAN and then RICH for a calibrated number of cycles while monitoring the OSC.

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