Catalyst efficiency on Lexus — IS/RX V6/V8 cat ageing, rear O2 sensor failures common (shared Toyota platform).
Lexus P0420 — Catalyst efficiency on Lexus — IS/RX V6/V8 cat ageing, rear O2 sensor failures common (shared Toyota platform). Typical UK independent garage cost: £180–£1500.
What actually causes P0420 on a Lexus
Lexus P0420 follows the Toyota story — rear O2 sensors slow with age and false-trigger P0420 long before the cat actually fails. Always swap a fresh Denso rear sensor first. On 2GR-FE V6, watch for bank identification (P0420 vs P0430) — both banks rarely fail together unless misfire history is involved. Hybrid Atkinson engines (2AR-FXE) see fewer cat issues but rear O2s still slow.
Lexus engines that log P0420
- 2GR-FE 3.5 V6 (IS350/RX350)
- 1UR-FE 4.6 V8 (LS460)
- 2AR-FXE 2.5 hybrid (IS300h/RX450h)
How we diagnose P0420 on a Lexus
- Techstream: log front/rear O2 switching per bank
- Swap a known-good Denso rear O2 and re-test
- Bore-scope cat through O2 port
- On V6, check for P0430 — bank confusion costs customers £1000+
- Fit genuine Denso or Toyota OE cats — pattern parts trigger P0420 again within months
The common misdiagnosis to avoid
Lexus P0420 — fit a Denso rear O2 first. On V6, never order two cats unless both P0420 and P0430 are confirmed.
Lexus P0420 repair cost in the UK
Expect £180–£1500 including parts and labour at an independent garage. Main-dealer quotes on Lexus 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
Lexus cats are pricey at main dealer but Denso aftermarket through Toyota indies is half the price — always quote both options.
Codes usually seen alongside P0420 on Lexus
- P0430
- P0136
- P0141
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.