A catalytic converter — usually just called a “cat” — is the part of your exhaust system that turns the most harmful gases your engine produces into less harmful ones before they leave the tailpipe. Every petrol car sold new in the UK has needed one since 1993, and diesels carry their own version alongside other emissions equipment.
How it works
Inside the steel shell sits a honeycomb core called a monolith, made of ceramic or metal and coated with a fine layer of precious metals — platinum, palladium and rhodium. The honeycomb structure packs an enormous surface area into a small space, and when the exhaust gases pass over that coated surface at operating temperature, the metals act as a catalyst: they trigger chemical reactions without being used up themselves.
Three reactions happen at once in a modern three-way cat:
- Carbon monoxide (CO) — a poisonous gas — is oxidised into carbon dioxide.
- Unburned hydrocarbons (HC) — raw fuel that escaped combustion — are oxidised into carbon dioxide and water vapour.
- Nitrogen oxides (NOx) — a major cause of urban air pollution — are reduced back into nitrogen and oxygen.
To do this the cat must be hot — it only starts working properly at several hundred degrees Celsius, which is why it sits close to the engine and why short cold-start journeys are the dirtiest miles a car does.
Petrol and diesel cats are different
Petrol engines use the three-way converter described above. Diesel engines run with excess air, so they use an oxidation catalyst (often called a DOC or diesel cat) that deals with CO and hydrocarbons, working alongside other equipment — the diesel particulate filter that traps soot, and on newer vehicles an SCR system that treats NOx using AdBlue. That’s why our catalogue lists diesel and petrol catalysts as distinct part ranges: they are engineered for different jobs, and the right one is determined by your exact vehicle and engine.
Why cats need replacing
A catalytic converter has no moving parts and can last the life of the car — but it is vulnerable to what the engine sends into it. Engine misfires, oil burning, coolant leaks and failed sensors all damage the monolith, and physical knocks can crack it. When that happens the cat stops converting efficiently and the car will typically fail its MOT emissions test. Before buying a replacement, it’s worth understanding why the old one failed — because if the underlying cause isn’t fixed, the new cat will go the same way.
Getting the right replacement
Replacement cats are specific to your vehicle, engine and its emissions standard — a part that physically fits is not necessarily approved for your car, and UK law requires type-approved replacements on most vehicles. The quickest way to find the right part is our part finder — select your manufacturer, model and engine and we’ll show you exactly what fits. Or browse the full catalytic converter range.
Find the part: browse petrol and diesel catalytic converters, combined catalytic converter and DPF units and SCR and DPF units, or go straight to your vehicle: Ford Focus catalytic converters, Ford Fiesta catalytic converters, Peugeot 208 catalytic converters, Vauxhall Astra catalytic converters or all vehicles.