Catalytic Converter Diagnostic

Catalytic Converter Diagnostic

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.

Two heated oxygen sensors are used to check the TWC, one upstream and one downstream of the catalytic converter. To determine catalytic converter efficiency the amplitude of the heated oxygen sensor signals is compared. During the catalytic converter diagnostic a controlled lean/rich fuel/air mixture cycle is carried out.

When TWC's efficiency is good and the fuel/air mixture is normal, the front heated oxygen sensor signal switches between rich and lean (with a large amplitude) while the rear heated oxygen sensor signal is even (has a small amplitude). When the TWC efficiency is poor and the fuel/air mixture is normal, the rear heated oxygen sensor signal switches between rich and lean (large amplitude).

When the amplitude ratio between the upstream and the downstream sensors exceeds a certain limit for a number of diagnostic tests, a DTC for "TWC efficiency reduced", is stored.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.