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.

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

1. Read pre- and post-cat O2 trims with VCDS
2. Check oil level vs last service — log consumption rate over 1,000 miles if suspect
3. Inspect PCV system — failed PCV diaphragm dumps oil into intake on EA888
4. 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.

Reading P0420 on a Audi properly

A generic reader will show P0420 and nothing else. What matters is the freeze-frame data stored with it — engine speed, load, coolant temperature and fuel trims at the moment the fault was logged — plus whether any other module on the Audi has logged something at the same time. Codes rarely arrive alone, and the order they were stored in usually points at which one is the cause and which are the consequences.

Clearing P0420 without fixing it

Erasing the code resets the Audi's readiness monitors as well as the light. The fault returns on the next drive cycle if the cause is still there, and in the meantime the incomplete monitors can stop the car passing an emissions check. After a genuine repair, the car is driven until the relevant monitor completes before it is handed back.

Bob's Mechanical Repairs is an independent garage in Birnam, Dunkeld, Perthshire, with 32 years of Bob's experience on the tools. If P0420 is showing on your Audi, bring any scan results with you — we will read the live data ourselves, confirm which cause it actually is, and price the repair before any work starts.