P0420 — "catalyst system efficiency below threshold, bank 1" — is the most misunderstood code in the book. Nine drivers out of ten are told they need a catalytic converter, and a fair share of them do not. All the code actually says is that the sensor after the cat is reading too much like the sensor before it, which means the cat is not scrubbing the exhaust as well as the ECU expects. A tired cat causes that. So does an exhaust leak, a lazy oxygen sensor, an old misfire, or a car that only ever does short cold trips. Working out which one you have before you spend £500 is the whole game.
What P0420 really means
Your car has two oxygen (lambda) sensors on each bank: one before the catalytic converter and one after it. The front one measures how rich or lean the engine is running. The rear one exists purely to check the cat is doing its job. On a healthy car the rear sensor produces a slow, almost flat voltage trace, because the cat has soaked up and converted the oxygen swings. When the cat wears out, the rear trace starts mirroring the front one, the ECU compares them, and after two consecutive drive cycles it stores P0420 and lights the engine management lamp. That is all the code proves: the two signals look too similar. It does not identify the cause.
The five real causes, cheapest first
1. **Exhaust leak before or between the sensors.** A blown manifold gasket, a cracked flexi pipe or a corroded joint lets fresh air in, skews the readings and throws P0420 with a perfectly good cat. Cost: £80 to £300. 2. **Lazy rear oxygen sensor.** Sensors slow down with age and mileage rather than failing outright. A 100,000-mile sensor can be technically alive but too slow to give a valid comparison. Cost: £90 to £220 fitted. 3. **Unresolved misfire or rich running.** A car that has been misfiring, running rich or burning oil has been poisoning the cat's honeycomb. Fix the cause first or the new cat goes the same way. Cost: varies. 4. **Short-journey driving.** A cat needs to reach 400°C to work. A car that only does three-mile school runs never gets there and slowly coats its honeycomb in unburnt deposits. 5. **Genuinely failed catalytic converter.** Internally melted, broken up, or rattling when you tap it. This is the real thing, and it is the last conclusion, not the first. Cost: £250 to £1,200 fitted depending on car.
How to test it properly before spending
Any decent UK garage can do this in under an hour, and it is worth insisting on: - **Live data on both lambda sensors.** Hold 2,500rpm and watch the rear trace. If it swings in step with the front, the cat is genuinely poor. If it stays lazy and flat, the cat is fine and the code has another cause. - **Smoke test or visual on the exhaust.** Leaks are audible cold and visible on a lift. - **Check fuel trims.** Long-term trims over about ±10 per cent mean the engine has a fuelling problem that is killing the cat. - **Tap test.** A dead cat often rattles when you tap the body with a rubber mallet — the internal substrate has broken up. - **Back-pressure test.** A blocked cat shows high back-pressure and the car feels gutless above 3,000rpm. Ask for a diagnostic report, not just a code read. Expect to pay £50 to £90 for it, and expect that to be the best money you spend on this fault.
MOT and legality in the UK
P0420 on its own is not an automatic MOT failure, but the consequences usually are. The engine management light being illuminated is a fail in its own right on cars first used from 2018 onwards, and a worn cat will very often push CO and hydrocarbon readings over the emissions limit on any car. Removing the cat entirely, or fitting a hollowed-out 'test pipe', is illegal for road use and an instant fail on the visual check. A cheap aftermarket cat is also a common cause of P0420 coming straight back within a few thousand miles. UK-market pattern cats vary enormously in the amount of precious metal in the substrate. If you buy one, buy a type-approved unit with an 'E' mark, not the cheapest on an auction site.
What UK garages charge
| Job | Typical fitted price | |---|---| | Full diagnostic with live lambda data | £50 – £90 | | Exhaust manifold gasket | £120 – £300 | | Flexi pipe repair | £90 – £220 | | Rear oxygen sensor | £90 – £220 | | Front oxygen sensor | £110 – £260 | | Aftermarket catalytic converter | £250 – £600 | | OE catalytic converter | £600 – £1,200+ | On some cars — many VAG, BMW and Ford models — the cat is built into the exhaust manifold, which is why the same fault can be £300 on one car and £1,100 on another.
Should you just clear it and keep driving?
Honestly, plenty of people do, and on an older car it can be a defensible decision — a P0420 with no drivability problem is not going to leave you at the roadside. But be clear about what you are accepting. The engine light will stay on, so it masks the next fault. Your MOT emissions result is on borrowed time. And if the cause is actually a rich-running engine, you are gradually washing the bores and diluting the oil. The sensible middle ground: get the diagnostic done, fix anything cheap that shows up (leak, sensor, misfire), do a couple of long motorway runs, then see whether the code returns after three or four drive cycles. If it does, and the live data says the cat is dead, budget for the cat.
Can I drive with a P0420 code? — Yes, generally. P0420 rarely affects drivability — the car will drive normally with the light on. It matters for your MOT emissions test and it hides any new fault behind an already-lit warning lamp, so it should be diagnosed rather than ignored indefinitely.
Does P0420 always mean a new catalytic converter? — No, and that is the most expensive assumption in the code. Exhaust leaks, a slow rear oxygen sensor and an underlying misfire all produce P0420 with a healthy cat. Ask for live lambda data before agreeing to a cat.
How much does it cost to fix P0420 in the UK? — Anywhere between £90 and £1,200. An oxygen sensor is £90 to £220, an exhaust leak £90 to £300, an aftermarket cat £250 to £600 and an OE cat on a manifold-integrated car £600 to £1,200 or more.
Will P0420 fail an MOT? — The code itself is not a listed failure, but the illuminated engine management light is a fail on cars first used from 2018, and a genuinely worn cat will usually push the emissions readings over the limit on any car.
Do catalytic converter cleaners work for P0420? — Occasionally, on a marginal cat that is coated in deposits from short-journey driving — used with a long, hot motorway run. They will do nothing for a cat with a collapsed internal substrate, an exhaust leak or a failed sensor.
How long can I drive with a bad catalytic converter? — If it is simply inefficient, indefinitely in practical terms. If it is breaking up internally, the debris can block the exhaust and starve the engine — that shows up as poor power above 3,000rpm, and it should be dealt with quickly.
P0420 means the catalytic converter is not converting as efficiently as the ECU expects, or the rear lambda sensor is misreporting. A lambda sensor is £90 to £220 fitted; an aftermarket cat is £250 to £700. Fix any misfire or oil burning first or the new cat fails within months.
Answered by Bob — 32 years on the tools, DVSA-approved MOT centre, Birnam, Perthshire.