Catalyst efficiency below threshold on Subaru — boxer engine cat failure from oil consumption and misfire history.
Subaru P0420 — Catalyst efficiency below threshold on Subaru — boxer engine cat failure from oil consumption and misfire history. Typical UK independent garage cost: £400–£1600.
What actually causes P0420 on a Subaru
Subaru P0420 on EJ-series boxers is often the result of years of oil consumption clogging the cat substrate — the head gasket weeps oil into one bank, oil burns through, and cat efficiency drops. On FB engines it's typically genuine cat ageing. STI owners with aftermarket downpipes often see P0420 with no actual fault — a cat-back rear O2 spacer is the common (questionable) fix.
Subaru engines that log P0420
- EJ20/EJ25 NA boxer (Forester/Outback/Impreza)
- FB20/FB25 boxer
- EJ20/25 turbo (WRX/STI)
How we diagnose P0420 on a Subaru
- Log front vs rear O2 sensor switching with scope
- Inspect spark plugs for oil fouling — points to head gasket weep
- Test compression and leak-down on suspect bank
- Inspect cat with bore-scope through O2 port
- Quote cat + head gasket work together if oil consumption confirmed
The common misdiagnosis to avoid
Always check for oil consumption on EJ before quoting a cat — if it's burning oil, a new cat lasts a year and you're back to square one.
Subaru P0420 repair cost in the UK
Expect £400–£1600 including parts and labour at an independent garage. Main-dealer quotes on Subaru 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
EJ25 with a soft head gasket and P0420 needs both fixed — cat-only jobs on these boxers are throwing money away.
Codes usually seen alongside P0420 on Subaru
- P0421
- P0430
- 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.
Reading P0420 on a Subaru 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 Subaru 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 Subaru'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 Subaru, 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.