Peugeot P0420

Catalyst efficiency on Peugeot petrol — 1.2 PureTech and older 1.6 VTi often have lambda drift before genuine cat failure.

Peugeot P0420 — Catalyst efficiency on Peugeot petrol — 1.2 PureTech and older 1.6 VTi often have lambda drift before genuine cat failure. Typical UK independent garage cost: £140–£750.

What actually causes P0420 on a Peugeot

The 1.2 PureTech (newer) has had emissions-related software updates that affect lambda monitoring strategy — always check for outstanding software campaigns at Peugeot main dealer first (often free). On the 1.6 VTi (Prince engine, shared with Mini) the rear lambda is unreliable and the timing chain wear can affect emissions, so check chain wear too.

Peugeot engines that log P0420

• 1.2 PureTech (208, 2008, 308)
• 1.6 VTi (208, 308, Mini)
• 1.4 / 1.6 8v (older 207/308)

How we diagnose P0420 on a Peugeot

1. Check Peugeot dealer for outstanding software actions on VIN
2. Graph both lambdas on scan tool
3. On 1.6 VTi, perform timing chain wear check (chain rattle on cold start)
4. Replace rear lambda as a trial before condemning cat

The common misdiagnosis to avoid

If on a 1.2 PureTech, the dealer may apply a software update that fixes P0420 for free under campaign. Always ring them with your VIN before paying for parts.

Peugeot P0420 repair cost in the UK

Expect £140–£750 including parts and labour at an independent garage. Main-dealer quotes on Peugeot 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

On 1.6 VTi engines (the Prince, used in Mini and Peugeot), if you've got P0420 plus any timing chain rattle, budget for both — chain failure on this engine destroys the head.

Codes usually seen alongside P0420 on Peugeot

• 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 Peugeot 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 Peugeot 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 Peugeot'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 Peugeot, 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.