Catalyst efficiency on Audi petrol — on TFSI engines this is regularly the rear O2 sensor or oil consumption (cat-fouling), not the cat.
Audi P0420 — Catalyst efficiency on Audi petrol — on TFSI engines this is regularly the rear O2 sensor or oil consumption (cat-fouling), not the cat. Typical UK independent garage cost: £160–£1100.
What actually causes P0420 on a Audi
EA888 Gen 1/2 2.0 TFSI engines have a well-known oil consumption issue (worn piston rings) which oil-fouls the catalyst over time. The P0420 is then real — the cat is genuinely damaged. Always do an oil consumption test (500ml per 1,000 miles is excessive) before authorising cat replacement on these.
Audi engines that log P0420
- 2.0 TFSI EA113/EA888 (A4, A5, A6, Q5)
- 1.4 TSI (A1, A3)
- 3.0 TFSI (A6, A7, Q7)
How we diagnose P0420 on a Audi
- Read pre- and post-cat O2 trims with VCDS
- Check oil level vs last service — log consumption rate over 1,000 miles if suspect
- Inspect PCV system — failed PCV diaphragm dumps oil into intake on EA888
- Only replace cat once oil consumption is under control
The common misdiagnosis to avoid
Fitting a new cat to a 2.0 TFSI that consumes oil heavily is a £800 mistake — the new cat will be fouled inside 20,000 miles. Fix the oil consumption (PCV / piston rings) first.
Audi P0420 repair cost in the UK
Expect £160–£1100 including parts and labour at an independent garage. Main-dealer quotes on Audi 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.
Service campaign / TSB note
Audi/VW oil consumption TSB on EA888 Gen 1/2 — check VIN for any open campaigns.
Bob's workshop tip
If you've got an early EA888 burning oil, fitting the updated pistons/rings (TSB campaign in many markets) is £1,800–£2,400 but extends engine life by 100,000+ miles and saves the cat too.
Codes usually seen alongside P0420 on Audi
- P0430
- P0171
- P053F
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.