Catalyst efficiency on Honda — Civic and Jazz cats are a major UK theft target, especially the IMA hybrid variants.
Honda P0420 — Catalyst efficiency on Honda — Civic and Jazz cats are a major UK theft target, especially the IMA hybrid variants. Typical UK independent garage cost: £280–£1200.
What actually causes P0420 on a Honda
Honda Jazz cats are stolen so frequently in UK cities that insurance companies have published warnings. P0420 with sudden loud exhaust = stolen cat. Genuine wear is also common past 120,000 miles. Honda OE cats are eye-wateringly expensive — type-approved aftermarket is generally fine for Hondas (better than Toyota in this respect).
Honda engines that log P0420
- Civic (8th gen FN, 9th gen FK)
- Jazz (GD, GE)
- Accord
- CR-V (RE5)
How we diagnose P0420 on a Honda
- Listen at startup — loud exhaust = theft
- Visual inspection under car for cut marks
- Tap test cat for substrate rattle
- Replace with type-approved direct-fit + cat protector
The common misdiagnosis to avoid
Always fit a cat protector / Catloc when replacing a Honda Jazz or Civic Hybrid cat — repeat theft is otherwise inevitable.
Honda P0420 repair cost in the UK
Expect £280–£1200 including parts and labour at an independent garage. Main-dealer quotes on Honda 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
Catloc / Armaloc protectors (£90–£200 fitted) pay for themselves the first time someone with an angle grinder gives up and walks away — insurers may also reduce premium if fitted.
Codes usually seen alongside P0420 on Honda
- 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.