Warm-up cat efficiency below threshold on Audi TFSI — manifold cat failure on EA888, often preceded by coil pack or wideband sensor drift.
Audi P0421 — Warm-up cat efficiency below threshold on Audi TFSI — manifold cat failure on EA888, often preceded by coil pack or wideband sensor drift. Typical UK independent garage cost: £400–£2100.
What actually causes P0421 on a Audi
A4 B8 2.0 TFSI: warm-up cat is manifold-integrated on some variants (expensive) and separate on others. Confirm before quoting. 3.0 TFSI S4 has two warm-up cats — bank 1 fails first typically.
Audi engines that log P0421
- 2.0 TFSI EA888 (A4 B8, A5)
- 1.4 TFSI EA211 (A1, A3)
- 3.0 TFSI EA837 (S4, S5)
How we diagnose P0421 on a Audi
- VCDS misfire counters and O2 activity
- Confirm cat/manifold integration on VIN
- Downstream sensor sweep test
- OE Audi cat/manifold if replacement needed
The common misdiagnosis to avoid
Assuming aftermarket manifold-cat fits — some A4 B8 have integral cats where the manifold and cat are one casting. Check part numbers before ordering.
Audi P0421 repair cost in the UK
Expect £400–£2100 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.
Bob's workshop tip
A4 B8 2.0 TFSI — check part numbers carefully. Some variants have manifold-integral cats.
Codes usually seen alongside P0421 on Audi
- P0420
- P0430
- P0300
- P0141
Symptoms drivers report with P0421
- Engine Management Light (EML) glowing amber on the dash.
- Noticing a slight 'rotten egg' sulfur smell during the first 5 minutes of driving.
- A rattling sound from the engine bay when you first pull away from cold.
- Passes the MOT emissions test when hot, but the light stays on anyway.
- Fuel economy dropping by 3 or 4 MPG as the ECU stays in 'warm-up' mode.
Repairs that clear P0421
- Cat replacement
- Sensor swap
For the generic, all-makes explanation of this code — meaning, every known cause and UK cost bands — see our P0421 guide.