Used catalytic converters: buying, fitting and the MOT

Signs your cat needs replacing

• An MOT emissions failure, or an engine warning light with a code pointing at catalyst efficiency.
• A rattle from under the car (the honeycomb inside has broken up), or a rotten-egg smell.
• A sudden loud roar from the exhaust when you start the car: the cat may have been stolen.

Getting the right one

• Match the part number stamped on the cat or its heat shield, not just the make and model. The same model often had different cats for different engines and emissions standards.
• Give the seller your registration and engine code: petrol and diesel versions, and Euro 4, 5 and 6 versions, aren't interchangeable.
• On diesels, the cat is often built into the same unit as the diesel particulate filter (DPF). Removing a DPF is illegal and an MOT fail, so replace like with like.
• Check the flanges, sensor bungs and brackets on a used one aren't cracked or rusted through.

Used, or a new replacement?

A used original-equipment cat from a low-mileage car usually works as well as new for far less. A new aftermarket replacement must be type-approved for your car (it will be marked with an E number); a cheap unapproved one can fail the emissions test or trigger warning lights.

Avoid buying a stolen cat

Catalytic converter theft is common because of the precious metals inside. Buy from a seller who can tell you which car the part came off, such as a breaker's yard that keeps records, and be wary of a cat offered with no history, sawn-off pipes or a price that looks too good. Every seller on Vehicle Part Finder is verified before they can list. Search for a catalytic converter for your car, or read buying used parts safely.