Dacia P0420

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

1. Live rear O2 switching read
2. Smoke-test exhaust for upstream leaks
3. Confirm no underlying misfire codes
4. 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.

Reading P0420 on a Dacia 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 Dacia 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 Dacia'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 Dacia, 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.