This is a practical, step-by-step guide to replacing a catalytic converter, based on the manufacturer’s own fitting guidance. It assumes competent DIY skills or a professional workshop — exhaust work involves a raised vehicle, seized fasteners and (until it cools) serious heat. If any step is outside your comfort zone, have a garage do the job; poor fitting is a leading cause of early failure and can invalidate the warranty.
Before anything else: make sure you know why the old cat failed and that the cause has been fixed. And note the new unit’s serial number (six letters, pin-stamped on the body) before it goes on the car — you’ll need it for any warranty query and it can be unreadable once fitted.
Preparation
- Let the exhaust cool completely — catalytic converters run at several hundred degrees and hold heat for a long time.
- Disconnect the battery earth terminal, so removing lambda sensors can’t upset the ECU.
- Raise the vehicle on a ramp — never work under a car supported only on jacks, and fit the catalyst with the vehicle on a ramp rather than axle stands.
- Offer the new part up against the old one before you start: same flanges, same bends, same sensor positions. If it looks like it would need modifying to fit, stop — you almost certainly have the wrong part for the vehicle, and any alteration voids the warranty. Email us and we’ll check the fitment.
- Soak all nuts, bolts and clamps in penetrating oil — ideally the day before.
Removal
- Photograph the installed unit from a couple of angles — a reference for routing and orientation later.
- Identify any lambda/oxygen sensors on the old unit. Unplug their connectors, then unscrew the sensors carefully — they seize readily and are easily damaged. Keep them somewhere protected.
- Loosen all retaining nuts, bolts and clamps. Seized fittings may need more penetrating oil or careful heat (with appropriate precautions).
- Support the rest of the exhaust so nothing hangs on the manifold, then remove the old unit — get a second pair of hands if it’s large or heavy.
Fitting
- Clean all mating surfaces back to sound metal and remove old gasket residue — air leaks at the joints ahead of a cat cause false lambda readings, MOT failures and premature cat death.
- Use a new fitting kit wherever possible: new gaskets, clamps, nuts and bolts. Old fixings lose thread and length to corrosion.
- Never use exhaust assembly paste on any joint before the cat. Hardened paste breaks up and destroys the monolith from the inside — and it’s an automatic warranty exclusion. If a sealant is genuinely needed, use a silicone-based one.
- Hang the new unit loosely — all fixings started but nothing tight — and align it with the rest of the system.
- Never strike the unit with a hammer or mallet to persuade it into position; the ceramic core will fracture.
- Refit the lambda sensors to the manufacturer’s torque — snug, not brutal — and reconnect their wiring, keeping cables clear of hot surfaces.
- Check clearance all round: the unit must not touch the floor pan, heat shields or anything else once the engine rocks under load.
- Tighten everything fully, working from the manifold back. Take care not to over-tighten.
After fitting
- Reconnect the battery, start the engine and check every joint for leaks — listen, and feel by hand around (not on) the joints. Re-lift the vehicle if needed.
- Clear any stored fault codes so the ECU starts fresh.
- Avoid hard driving for the first few miles while everything beds in, then re-check the fixings after a heat cycle if you can.
Need the right part first? Use the part finder or browse catalytic converters. Fitting a DPF instead? That job has extra, non-optional steps — see fitting a DPF.
Find the part: browse catalytic converters, front and connecting pipes and fitting kits, or go straight to your vehicle: Ford Focus catalytic converters, Volkswagen Golf catalytic converters or all vehicles.