Nissan P0420

Catalyst efficiency on Nissan petrol — the front cat-back O2 sensor is the weakness on Micra K12, Note E11 and Qashqai J10.

Nissan P0420 — Catalyst efficiency on Nissan petrol — the front cat-back O2 sensor is the weakness on Micra K12, Note E11 and Qashqai J10. Typical UK independent garage cost: £180–£700.

What actually causes P0420 on a Nissan

Nissan front cat assemblies on these models are well known to crack internally — the substrate loosens and rattles when tapped. P0420 with a rattle when you tap the cat = replacement needed. Without rattle, try a rear lambda first.

Nissan engines that log P0420

• Micra K12 1.2 / 1.4
• Note E11 1.4 / 1.6
• Qashqai J10 1.6 petrol
• Juke F15 1.6 DiG-T

How we diagnose P0420 on a Nissan

1. Tap test the cat with a rubber mallet — internal rattle = substrate failure
2. Graph both lambdas on scan tool
3. Trial rear lambda replacement if no rattle
4. Buy type-approved direct-fit cat — universal cats fail MOT on most modern Nissans

The common misdiagnosis to avoid

Aftermarket universal cats (£90–£140) generally don't pass MOT emissions on UK Nissan petrols — buy a type-approved direct-fit unit (£280–£480) to avoid a second MOT failure.

Nissan P0420 repair cost in the UK

Expect £180–£700 including parts and labour at an independent garage. Main-dealer quotes on Nissan 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

For MOT compliance always insist on a type-approved (e-marked) cat — keep the box and paperwork in the car for emission-test pushback.

Codes usually seen alongside P0420 on Nissan

• P0430
• P0171

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 Nissan 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 Nissan 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 Nissan'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 Nissan, 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.