Catalyst efficiency below threshold on Dacia — Renault-shared 1.5 dCi K9K and 0.9 TCe cat aging.
Dacia P0420 — Catalyst efficiency below threshold on Dacia — Renault-shared 1.5 dCi K9K and 0.9 TCe cat aging. Typical UK independent garage cost: £150–£550.
What actually causes P0420 on a Dacia
Dacia P0420 reads as a Renault problem — same K9K and TCe engines share the same cats and the same failure pattern. Cheap aftermarket cats on K9K are fine if E-marked; performance and emissions hold up well.
Dacia engines that log P0420
- 1.5 dCi K9K (Sandero/Duster/Logan)
- 0.9 TCe H4B (Sandero/Duster)
- 1.6 SCe (Duster)
How we diagnose P0420 on a Dacia
- Live rear O2 switching read
- Smoke-test exhaust for upstream leaks
- Confirm no underlying misfire codes
- Fit E-marked aftermarket and re-MOT
The common misdiagnosis to avoid
Dacia owners are price-sensitive — quote a quality aftermarket cat with E-mark first, only OEM if the customer prefers.
Dacia P0420 repair cost in the UK
Expect £150–£550 including parts and labour at an independent garage. Main-dealer quotes on Dacia 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
Dacia Duster P0420 — perfect candidate for a £180 aftermarket cat. Customer keeps a workhorse cheap.
Codes usually seen alongside P0420 on Dacia
- P0430
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.