The catalytic converter is a key emissions control device in petrol vehicles, converting harmful exhaust gases into less harmful substances before they leave the tailpipe.
How It Works
A catalytic converter contains a ceramic honeycomb coated with precious metals (platinum, palladium, rhodium). Hot exhaust gases pass through and react with these catalysts. Carbon monoxide (CO) is converted to CO₂, unburned hydrocarbons to CO₂ and water, and nitrogen oxides (NOx) to nitrogen gas. The process only works effectively above approximately 300°C — hence the need to drive gently when the engine is cold.
Diesel Particulate Filter (DPF)
Diesel vehicles use a DPF to trap soot particles. The filter regenerates periodically (burns off trapped particles) during motorway-speed driving. Short city trips prevent regeneration — the filter fills up and triggers a warning light. If ignored, the DPF can require expensive replacement.
What Damages the Catalyst
Leaded fuel (historically), engine oil burning (worn engine), using the wrong fuel, and repeated cold starts all degrade the catalyst. A damaged catalyst increases emissions significantly and will fail the annual inspection.
Exam tip: A DPF warning light in a diesel car means the filter needs a regeneration run — typically a 20-minute motorway drive at sustained speed. Ignoring it leads to expensive repairs.