Honda P0420

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

1. Listen at startup — loud exhaust = theft
2. Visual inspection under car for cut marks
3. Tap test cat for substrate rattle
4. 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.

Reading P0420 on a Honda 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 Honda 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 Honda'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 Honda, 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.