Catalyst efficiency on VW — on EA888 TSI engines, oil consumption damages the cat genuinely. On older EA113 it's frequently a rear lambda drift.
Volkswagen P0420 — Catalyst efficiency on VW — on EA888 TSI engines, oil consumption damages the cat genuinely. On older EA113 it's frequently a rear lambda drift. Typical UK independent garage cost: £150–£1100.
What actually causes P0420 on a Volkswagen
The 1.4 TSI (twincharger and later) suffers timing chain stretch — a stretched chain causes intermittent misfires, which damage the cat and store P0420. Always check chain wear on a 1.4 TSI before condemning the cat. On 2.0 TSI EA888 it's oil consumption (same story as Audi A4 2.0 TFSI).
Volkswagen engines that log P0420
- 1.4 TSI (Golf VI/VII, Polo, Tiguan)
- 2.0 TSI EA888 (Golf GTI, Passat, Tiguan)
- 1.8 TSI (Passat B6/B7, Tiguan)
How we diagnose P0420 on a Volkswagen
- Read freeze frame and misfire counters — chain stretch shows as intermittent cylinder misfires
- Test rear lambda — replace if drifted
- On 2.0 TSI, oil consumption test
- Only replace cat once underlying mechanical cause is resolved
The common misdiagnosis to avoid
Don't replace the cat on a 1.4 TSI with a stretched timing chain — even a brand new cat will be damaged by ongoing misfires.
Volkswagen P0420 repair cost in the UK
Expect £150–£1100 including parts and labour at an independent garage. Main-dealer quotes on Volkswagen 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
Quality aftermarket cats (BM Cats / Klarius) at around £180–£280 are perfectly fine on these — the OEM VW cat at £600+ is rarely worth the premium for a 5+ year old car.
Codes usually seen alongside P0420 on Volkswagen
- P0430
- P0300
- P0171
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.