Catalytic Converter: Description and Operation

PURPOSE
The control module must monitor the Three-Way catalyst system (TWC) for efficiency. In order to accomplish this, the control module monitors the pre-catalyst and post-catalyst oxygen sensors. When the TWC is operating properly, the post-catalyst (2) oxygen sensor will have significantly less activity than the pre-catalyst (1) oxygen sensor. The TWC stores oxygen as needed during its normal reduction and oxidation process. The TWC releases oxygen as needed during its normal reduction and oxidation process. The control module calculates the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor's voltage levels.

OPERATION
Whenever the voltage levels of the post-catalyst (2) oxygen sensor nears the voltage levels that of the pre-catalyst (1) oxygen sensor, the catalysts efficiency is degraded.

Stepped or staged testing levels allow the control module to statistically filter test information. This prevents falsely passing or falsely failing the oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. For this reason, do not use post catalyst oxygen sensor activity in order to determine the oxygen storage capacity unless directed by the electronic service information

Three stages are used in order to monitor catalyst efficiency. Failure of the first stage indicates that the catalyst requires further testing in order to determine catalyst efficiency. Failure of the second stage indicates that the catalyst may be degraded. The third stage then looks at the inputs from the pre and post O2 sensors more closely before determining if the catalyst is indeed degraded. This further statistical processing is done to increase the accuracy of oxygen storage capacity type monitoring. Failing the first (stage 0) or second (stage 1) test Does Not indicate a failed catalyst. The catalyst may be marginal or the fuel sulfur content could be very high.

Aftermarket HO2S characteristics may be different from the original equipment manufacturer sensor. This may lead to a false pass or a false fail of the catalyst monitor diagnostic. Similarly, if an Aftermarket catalyst does not contain the same amount of cerium as the original part, the correlation between oxygen storage and conversion efficiency may be altered enough to set a false DTC.
The EVAP system uses a switch located in the purge line between the canister and the purge valve in order to detect when the purge is occurring. This switch senses the flow from the engine through the purge valve. When no purge is present, the switch is closed, applying a 12 volt signal to the control module as a NO PURGE signal. When canister purging occurs, the switch opens, interrupting off the 12 volt signal to the control module. A scan tool display will indicate that purge is occurring.

Clogging of the canister fresh air vent could allow the purge hose between the switch and canister to trap vacuum with the purge valve closed. This would result in a diagnostic indication of a purge valve stuck open or a vacuum switch failure. Similarly, leaks or blockages in the purge hoses may result in misdiagnosis of the purge valve or vacuum switch.

When servicing a purge valve diagnostic trouble code, check the canister fresh air vent, vacuum switch and the integrity of all purge hoses prior to servicing the valve.