Lexus P0420

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

1. Techstream: log front/rear O2 switching per bank
2. Swap a known-good Denso rear O2 and re-test
3. Bore-scope cat through O2 port
4. On V6, check for P0430 — bank confusion costs customers £1000+
5. 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.

Reading P0420 on a Lexus 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 Lexus 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 Lexus'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 Lexus, 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.