Catalyst efficiency on Jaguar — AJ-V8 5.0 NA/SC and AJ200 2.0 Ingenium petrol cat issues.
Jaguar P0420 — Catalyst efficiency on Jaguar — AJ-V8 5.0 NA/SC and AJ200 2.0 Ingenium petrol cat issues. Typical UK independent garage cost: £600–£2800.
What actually causes P0420 on a Jaguar
Jaguar P0420 on the 5.0 V8 is normally melted cat substrate from misfire history — coil packs and plugs aren't a routine service item but should be on these. On AJ200 2.0 Ingenium, cat ageing around 80k is common and twin-bank engines often only fail one side. Always confirm which bank with P0420 vs P0430 before quoting — single cat replacement is half the price.
Jaguar engines that log P0420
- AJ133 5.0 V8 NA/SC (XF/XJ/F-Type)
- AJ200P 2.0 Ingenium petrol
- AJ126 3.0 V6 SC
How we diagnose P0420 on a Jaguar
- SDD/Pathfinder: log front/rear O2 switching per bank
- On V8, inspect coil packs and plugs — misfire history damages cats
- Bore-scope cat substrate through O2 port
- Check for any DTCs on the opposite bank — bilateral failure changes the quote
- Quote OE or Eberspächer aftermarket — bargain cats fail Euro 5+
The common misdiagnosis to avoid
Always identify P0420 (bank 1) vs P0430 (bank 2) on Jaguar V6/V8 — fitting both cats when only one is failed wastes £1000+.
Jaguar P0420 repair cost in the UK
Expect £600–£2800 including parts and labour at an independent garage. Main-dealer quotes on Jaguar 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
AJ133 V8 with P0420 and any misfire DTC needs the misfire root cause fixed first — a fresh cat survives weeks if a coil keeps dumping fuel into it.
Codes usually seen alongside P0420 on Jaguar
- P0430
- P0300
- 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.