A catalytic converter \u2014 usually just called a „cat“ \u2014 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.<\/p>\n
Inside the steel shell sits a honeycomb core called a monolith<\/strong>, made of ceramic or metal and coated with a fine layer of precious metals \u2014 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.<\/p>\n Three reactions happen at once in a modern three-way cat:<\/p>\n To do this the cat must be hot \u2014 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.<\/p>\n Petrol engines use the three-way converter described above. Diesel engines run with excess air, so they use an oxidation catalyst<\/strong> (often called a DOC or diesel cat) that deals with CO and hydrocarbons, working alongside other equipment \u2014 the diesel particulate filter<\/a> 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.<\/p>\n A catalytic converter has no moving parts and can last the life of the car \u2014 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<\/a> \u2014 because if the underlying cause isn’t fixed, the new cat will go the same way.<\/p>\n Replacement cats are specific to your vehicle, engine and its emissions standard \u2014 a part that physically fits is not necessarily approved for your car, and UK law requires type-approved<\/a> replacements on most vehicles. The quickest way to find the right part is our part finder<\/a> \u2014 select your manufacturer, model and engine and we’ll show you exactly what fits. Or browse the full catalytic converter range<\/a>.<\/p>\n Find the part:<\/strong> browse petrol and diesel catalytic converters<\/a>, combined catalytic converter and DPF units<\/a> and SCR and DPF units<\/a>, or go straight to your vehicle: Ford Focus catalytic converters<\/a>, Ford Fiesta catalytic converters<\/a>, Peugeot 208 catalytic converters<\/a>, Vauxhall Astra catalytic converters<\/a> or all vehicles<\/a>.<\/p>\n\n
Petrol and diesel cats are different<\/h2>\n
Why cats need replacing<\/h2>\n
Getting the right replacement<\/h2>\n