What are the signs a catalytic converter might be failing?
The usual signs are a noticeable drop in power or acceleration, worse fuel economy than normal, a rattling sound from underneath the car (often more obvious at idle or low revs), a smell like rotten eggs or sulphur from the exhaust, and an engine management light that stays on. Any one of these can also point to something else entirely, which is why none of them is a reliable diagnosis on its own.
Does a rattling noise always mean the catalytic converter is failing?
Not always. A rattle from underneath the car can also come from a loose heat shield, a worn exhaust mount, or something caught in the exhaust system — all of which sound similar to a driver but are very different jobs to fix. The only way to tell them apart is to get the car up on a ramp and have a proper look. If it's the engine light that's really worrying you rather than a noise, see our guide on the engine management light.
Can I tell from the engine management light alone?
No. The engine management light covers a huge range of faults, from something as minor as a loose fuel cap to something as involved as a failing converter. It needs a proper diagnostic check to read the fault codes and work out what's actually triggering it, rather than guessing from the light alone.
What causes a catalytic converter to fail?
Catalytic converters are built to last a long time, but they can be damaged by driving with an engine that's already misfiring or burning oil, by a damaged sensor further up the exhaust, or simply by age and mileage. Because the converter reacts to problems elsewhere in the engine, it's worth finding out whether something else caused it to fail, not just replacing it and hoping.
How is a catalytic converter diagnosed properly?
It starts with a £48.00 inc. VAT diagnostic check to read the fault codes and see what the engine is actually doing, followed by a look at the exhaust and the sensors around the converter itself. Guesswork from the symptoms alone isn't enough, because several different faults can look identical from the driver's seat. See our guide on what an engine management diagnostic costs for how that check works.
Can a catalytic converter issue come back after it's fixed?
If whatever caused it in the first place — a misfire, a worn sensor, an engine burning oil — isn't dealt with as well, a replacement can fail again. That's why it matters to look at the whole picture rather than just swapping the one part.
How much does it cost to replace a catalytic converter?
There's no single figure for this, because it depends on the vehicle and on whether anything else needs fixing alongside it. The sensible first step is the diagnostic check above to find out exactly what's going on, after which you'll get a written quote before anything is replaced.
What happens if I bring it to ONEFIX?
Every check at ONEFIX includes a written digital vehicle health check with photographs, so you can see what's actually wrong rather than just being told. We quote any work in writing before it starts, and nothing is carried out without that quote being approved.
Is it safe to keep driving with a suspected failing catalytic converter?
It depends on the symptoms, so this isn't something to guess at from a description alone. A persistent engine management light or a sudden loss of power is worth getting checked promptly rather than left to see if it gets worse.
What should I do next?
If you're seeing any of these signs, book a diagnostic check at ONEFIX, Plas Acton Garage, Chester Road, Wrexham. It's the quickest way to find out whether it really is the catalytic converter, or something else causing the same symptoms.