Catalytic Converter

Inside the steel shell is a honeycomb coated with precious metals such as platinum, palladium and rhodium. On petrol engines a three-way catalyst reduces carbon monoxide, unburnt hydrocarbons and nitrogen oxides at the same time, as long as the mixture is kept very close to stoichiometric.

Diesels use an oxidation catalyst instead, usually combined with a particulate filter and SCR system for NOx. OBD-II monitors the catalyst’s efficiency with an oxygen sensor after it; low efficiency sets codes such as P0420 or P0430. Petrol cars use a three-way catalyst that reduces carbon monoxide, hydrocarbons and nitrogen oxides at the same time. Diesel cars use an oxidation catalyst, combined with a particulate filter and often an SCR system for NOx.

The coating speeds up chemical reactions in the hot exhaust. A working catalyst stores and releases oxygen, which smooths out the downstream oxygen sensor signal; the control unit uses that to judge its health. Inside is a ceramic or metal honeycomb coated with precious metals such as platinum, palladium and rhodium. It only works properly once it is hot, which is why cold starts produce most emissions.

Misfires and an engine burning oil can overheat or poison the catalyst. A failing catalyst means higher emissions, a failed inspection and sometimes a rattle, a rotten-egg smell or loss of power if it is blocked. Misfires, burning oil or coolant can overheat or poison a catalyst. Replacing it is expensive, so the cause must be fixed first; otherwise a new converter can fail just as quickly.