P0420 means the catalytic converter on bank 1 is not cleaning the exhaust as well as the engine computer expects, or the sensor watching it is misreading. It is not an emergency, but it will not clear itself, and the warning light on its own is an MOT failure. Indicative UK repair cost is £60 to £900.
It is the single most-Googled engine code, and it is also the one most often fixed by replacing the wrong part. P0420 is a judgement the engine computer makes by comparing two oxygen sensors, one in front of the converter and one behind it. Anything that upsets that comparison will set the code, and only some of those things are a dead converter.
Can I drive with P0420?
Diagnose soon, drive gently. The car will usually drive normally and you can keep using it, but the fault will not clear itself, and a converter that is breaking up inside can block and take your power with it. If the light starts flashing, or misfire codes appear alongside it, stop treating this as a catalyst problem and treat it as urgent. Raw petrol passing through a hot converter is what destroys them.
What P0420 means for your MOT
Three separate things can fail here, and they are worth keeping apart.
- The light itself. Section 8.2.1.2 of the DVSA MOT inspection manual lists "engine MIL inoperative or indicating a malfunction" as a major defect. The light is inspected on petrol cars and petrol hybrids first used on or after 1 July 2003, and on diesels first used on or after 1 July 2008. On those cars an illuminated engine management light fails the test on its own, whatever the stored code says.
- The emissions reading. A converter that has genuinely gone will often push hydrocarbons or the lambda figure outside the permitted limits at the tailpipe, and that is a separate major defect under the same section.
- The converter as a part. If it has been removed, hollowed out or replaced with a decat, that is a major defect under section 8.2.1.1, which covers manufacturer-fitted emissions equipment that is missing, obviously modified or obviously defective.
Clearing the code the day before the test does not get you through it. The light returns as soon as the car has completed the drive cycle that monitor needs, and on some cars that is a single journey.
What usually causes it
- An ageing or failing catalytic converter. The honeycomb inside stops holding oxygen the way it did when new. High mileage brings this on, and so does a life of short urban journeys where the converter never gets properly hot.
- A faulty downstream (rear) oxygen sensor. The code is a comparison, so a lazy rear sensor reads exactly like a lazy converter. This is one of the cheapest causes and one of the commonest.
- An exhaust leak before the cat. A split flexi pipe or a leaking manifold joint lets air into the system and skews what both sensors see.
- An unresolved misfire, or a mixture running rich. Unburnt petrol reaching the converter overheats it and shortens its life. Fixing that after the converter has been replaced is the wrong way round.
- A pattern or universal replacement converter. Worth adding on UK cars, because so many have had one fitted after a catalytic converter theft. Cheap aftermarket converters and sports cats often set P0420 within a few hundred miles, because they do not hold enough precious metal to pass the engine computer's own test even when nothing is wrong with the engine.
What it costs to fix in the UK
£60 to £900. The spread is real rather than hedging. A rear oxygen sensor sits near the bottom of it, an exhaust leak repair in the middle, and a genuine converter at the top, with V6 and V8 cars higher again because there are two of them.
A garage diagnostic fee sits on top of all of that, so it is worth knowing what a car diagnostic costs in the UK before you book one in.
These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.
What to check first
- Read every stored code, not just this one. If misfire codes (P0300 to P0304) or fuel trim codes (P0171, P0172) are stored alongside it, they come first. A converter fitted under a live misfire fails again.
- Go underneath and listen. A rattle when you tap the converter means the honeycomb has broken up. A blow at the flexi pipe or the manifold joint means air is getting in. Both are cheaper to find than to assume.
- Compare the two sensors on live data. The front sensor should be switching quickly. The rear one should sit fairly flat. If the rear sensor is copying the front, the converter is not buffering oxygen. If the rear sensor is slow and the front one is fine, suspect the sensor.
- Ask what has already been fitted. A universal cat, a sports cat or a decat will set this code on a perfectly healthy engine, and no amount of further diagnosis changes that.
Related codes
- P0300, random or multiple cylinder misfire. The most common reason a healthy converter dies young.
- P0442, P0455 and P0456, EVAP system leaks. Also emissions codes, and a useful comparison if you have more than one stored, because they carry nothing like the same risk.
P0430 is the same fault on the other bank of a V6 or V8, and P0171, running lean, often sits behind a converter that has failed early.
Get it narrowed down before you spend
A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.
Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.