Catalytic Converter Diagnostic

Catalytic Converter Diagnostic

General Description









The three-way catalytic converter (TWC) stores oxygen found in the exhaust gases and uses it to make toxic gases less dangerous. The catalytic converter is a TWC converter in which hydrocarbons (HC) and carbon monoxide (CO) are oxidized and oxides of nitrogen (NOx) are reduced.

As the TWC ages its ability to store oxygen drops. This reduces the conversion capacity of the TWC. To avoid dangerous emissions the ECM checks TWC efficiency. This check is carried out as follows.

The two-sensor method stereo makes use of one upstream and one downstream oxygen sensor for each cylinder bank, each bank has one sensor before the catalytic converter (UHEGO) and one after (HEGO).

Rich and lean lambda pulses are sent through the TWC. For a TWC with good gas converter and large oxygen storage capacity, it will take a long time for the rich/lean pulse to reach the rear oxygen sensor. The rear oxygen sensor will then have long rich and lean pulses and a long time between switches. When the TWC detoriate and oxygen storage capacity drops will the rear oxygen sensor switching frequency increase. The rear oxygen sensor voltage will be used to calculate a test value of the TWC performance and a malfunctioning TWC will be detected by OBD II system.