Catalyst efficiency below threshold on Citroën — PSA THP and PureTech petrols suffer cat degradation from oil burning.
Citroën P0420 — Catalyst efficiency below threshold on Citroën — PSA THP and PureTech petrols suffer cat degradation from oil burning. Typical UK independent garage cost: £240–£1300.
What actually causes P0420 on a Citroën
On 1.6 THP (Prince-derived, shared with Mini), oil consumption damages the cat over time — fix the oil burn before any cat work. On 1.2 PureTech, wet-belt failure putting debris through the engine eventually destroys the cat. The 1.6 VTi typically suffers genuine age-related cat wear past 100k.
Citroën engines that log P0420
- 1.6 THP (DS3 Racing, DS4)
- 1.2 PureTech turbo (C3, C4 Cactus, DS3)
- 1.6 VTi (C3, C4)
How we diagnose P0420 on a Citroën
- Check oil consumption — anything over 0.5L/1000 miles points at root cause work
- On 1.2 PureTech, pull rocker cover and inspect wet belt condition
- Diagbox: log pre and post-cat lambda voltages
- Inspect for upstream misfires (coil packs, plugs)
- Quote root-cause work BEFORE cat replacement
The common misdiagnosis to avoid
Don't fit a cat to a 1.6 THP burning a litre of oil per 1,000 miles or a PureTech with belt issues — sort the root cause first or the new cat will fail just as quickly.
Citroën P0420 repair cost in the UK
Expect £240–£1300 including parts and labour at an independent garage. Main-dealer quotes on Citroën 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
PureTech P0420 alongside oil consumption is the wet-belt warning — pull that rocker cover before the customer commits to a cat they don't actually need yet.
Codes usually seen alongside P0420 on Citroën
- P0430
- P0171
- P2096
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.