Scheme 511
Theory Of Operation
The catalyst monitor uses the signals from both the upstream and downstream O2 sensors to detect aging of the catalyst. As a catalyst ages, it loses some of its oxygen storing capacity. As a result, part of the untreated exhaust gases can break through the catalyst and causes the downstream O2 sensor to deviate from its neutral (Stoichiometric) position. By observing the downstream O2 signal, the degradation level of catalyst can be detected. In general, the higher the downstream O2 sensor state of change value, the more exhaust gas breakthrough and the lower the oxygen storage capacity of the catalytic converter.
The EVAP Purge Monitor tests the integrity of the hoses/tube between the throttle body/intake and the fuel tank. The monitor is a two stage test and runs only after the EVAP system passes the small leak test. Stage one is non-intrusive. The Powertrain Control Module (PCM) monitors the purge vapor ratio and the Evaporative System Integrity Monitor (ESIM) Switch closed ratio. If the purge vapor ratio is above a calculated value, the monitor passes. If the ESIM switch closed ratio is greater than calculated value when purge flow is greater than a minimum value, the monitor passes. Stage two is an intrusive test and runs only if stage one does not pass. The PCM commands the purge solenoid to flow at a specified rate to force the purge vapor ratio to update. The ratio is compared to a calibrated specification. If it is less than specified, a one trip failure is recorded. This test can detect if the purge hose is off, obstructed, or the purge valve is not operational.