Catalyst efficiency on Mercedes petrol — on M271 1.8 Kompressor, the supercharger oil seal failures fouls the cat. On M276 V6, lambda sensor drift is more common.
Mercedes-Benz P0420 — Catalyst efficiency on Mercedes petrol — on M271 1.8 Kompressor, the supercharger oil seal failures fouls the cat. On M276 V6, lambda sensor drift is more common. Typical UK independent garage cost: £180–£1400.
What actually causes P0420 on a Mercedes-Benz
On the M271 Kompressor the supercharger nose-cone oil seal leaks oil into the intake; that oil fouls the cat and stores P0420. On the M276 V6 cats are typically fine until 100k+ miles — earlier P0420 is usually a drifted post-cat lambda. Always graph both lambdas before replacing.
Mercedes-Benz engines that log P0420
- M271 1.8 Kompressor (C-Class W204, E-Class W212, SLK)
- M271 EVO direct injection
- M276 3.5 V6 (C350, E350, SLK350)
How we diagnose P0420 on a Mercedes-Benz
- Inspect intake for oil residue (Kompressor nose-cone seal leak)
- Graph pre- and post-cat lambdas on STAR / XENTRY
- Check for any stored misfire codes that may have damaged the cat
- Replace rear lambda first if cat function looks salvageable
The common misdiagnosis to avoid
M271 Kompressor cars with high mileage frequently have leaking supercharger oil seals — fit a new cat without fixing the supercharger and the new cat will fail within months.
Mercedes-Benz P0420 repair cost in the UK
Expect £180–£1400 including parts and labour at an independent garage. Main-dealer quotes on Mercedes-Benz are usually higher for the same job, and the spread exists because the cheap cause and the expensive cause throw the same code — pay for the diagnosis before you pay for parts.
Bob's workshop tip
Mercedes-spec aftermarket cats are widely available at £280–£450 — well below the genuine MB part. For older C-Class/E-Class out of warranty this is the sensible move.
Codes usually seen alongside P0420 on Mercedes-Benz
- P0430
- P0171
- P0174
Symptoms drivers report with P0420
- Engine warning light on steady, no obvious drivability change day-to-day
- 3–7 mpg drop over the last 2,000 miles with no other explanation
- Faint smell of sulphur / rotten eggs from the exhaust under load
- Muffled rattle inside the cat when you tap it with a rubber mallet (loose substrate)
- MOT emissions test fails on Lambda, CO or HC — the P0420 was already MIL-on at the pre-test
- Sudden loud exhaust roar from cold start = catalytic converter has been stolen, not worn
- Paired codes: P0430 (bank 2), P0171/P0174 (lean), P0300-series misfires, P0136/P0156 (lambda circuit)
Repairs that clear P0420
- Smoke-test exhaust for leaks
- Graph both lambda outputs at steady state
- Replace rear lambda as cheaper trial
- Replace cat with type-approved unit if confirmed
For the generic, all-makes explanation of this code — meaning, every known cause and UK cost bands — see our P0420 guide.