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
- Check Peugeot dealer for outstanding software actions on VIN
- Graph both lambdas on scan tool
- On 1.6 VTi, perform timing chain wear check (chain rattle on cold start)
- 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.