Catalyst efficiency below threshold on Renault — 1.2 TCe, 1.6 dCi rare, but 1.2 TCe is the headline case.
Renault P0420 — Catalyst efficiency below threshold on Renault — 1.2 TCe, 1.6 dCi rare, but 1.2 TCe is the headline case. Typical UK independent garage cost: £260–£1400.
What actually causes P0420 on a Renault
Renault 1.2 TCe (H5Ft engine) had widespread oil consumption issues from worn piston rings — the cat is the silent victim, killed by oil ash. P0420 on these usually means the oil burn has already damaged the cat. Replacing the cat without fixing rings means revisiting the job within 12 months.
Renault engines that log P0420
- 1.2 TCe (Clio, Megane, Captur)
- 1.6 16v (older Clio, Megane)
- 0.9 TCe 3-cyl turbo
How we diagnose P0420 on a Renault
- Check oil consumption — 1.2 TCe rings issue is well-documented
- Check Renault VIN for extended warranty on 1.2 TCe ring failures
- Scan tool: log pre and post-cat lambda voltages
- Inspect for upstream misfires
- Quote root-cause work before cat replacement
The common misdiagnosis to avoid
Don't fit a cat to a 1.2 TCe burning oil — Renault extended warranties on some 1.2 TCe engines for piston ring failure. Check VIN before the customer pays for cat work.
Renault P0420 repair cost in the UK
Expect £260–£1400 including parts and labour at an independent garage. Main-dealer quotes on Renault 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
Renault extended warranty on 1.2 TCe piston ring failures, UK market.
Bob's workshop tip
Renault honoured 1.2 TCe ring failures well outside standard warranty in the UK — always check the VIN before customers pay for cats they didn't need.
Codes usually seen alongside P0420 on Renault
- P0430
- P0171
- P2096
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.