Your catalytic converter isn't cleaning exhaust gases as well as it should — but check exhaust leaks and the rear lambda before condemning the cat.
P0420 — also written PO42O, P 0420, 0420 — is the OBD-II code for catalyst system efficiency below threshold (bank 1). Same code, same fix: this page covers every spelling of it.
Catalyst System Efficiency Below Threshold (Bank 1) — the second most-Googled UK fault code and the single biggest cause of MOT emissions failures. The ECU is telling you the rear (post-cat) oxygen sensor is swinging almost as much as the front one, which means the catalytic converter isn't scrubbing exhaust gases the way it should. Nine times out of ten it's NOT a dead cat — it's a lazy rear lambda, a small exhaust leak upstream, or an unresolved misfire silently killing the cat you've already got. On Toyota, Honda and Lexus hybrids in UK cities it is also very often catalytic converter theft, not wear.
P0420 is a emissions fault code, typically costing £180–£1400 to put right in a UK garage.
What P0420 actually feels like to drive
- 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)
What causes P0420?
- Worn catalytic converter
- Failed downstream (rear) O2 sensor
- Exhaust leak before the cat
- Underlying misfire damaging the cat
- Cat substrate cracked (rattle on tap test)
UK vehicles that throw P0420 most often
- Vauxhall Corsa / Astra / Insignia 1.2 / 1.4 / 1.6 petrol (A14NET especially) — rear lambda drift epidemic
- Ford Fiesta / Focus / Kuga 1.0 / 1.5 / 1.6 EcoBoost — cracked integrated exhaust manifold before the cat
- VW Polo / Golf / Audi A1 / A3 1.2 & 1.4 TSI — misfire from stretched chain damaging cat
- Toyota Yaris / Auris / Corolla / Prius / C-HR hybrid — genuine wear at 150,000+ miles AND catalytic converter theft
- Honda Jazz / Civic / CR-V (particularly Jazz hybrid) — #1 UK insurance-flagged theft target
- Nissan Micra / Note / Qashqai / Juke 1.2 / 1.5 / 1.6 petrol — front cat substrate cracking (rattles when tapped)
- Peugeot 208 / 2008 / 308 1.2 PureTech — dealer software campaign may fix P0420 free with VIN
- BMW 1 / 3 Series N20 / N26 — integrated manifold crack similar to Ford EcoBoost
- Mercedes A / B / C-Class M270 / M274 turbo petrol — rear lambda ageing before cat
- Kia Picanto / Rio / Ceed 1.0 / 1.2 / 1.4 petrol — cheap universal cats fitted at previous owner stage
How P0420 is fixed
- 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
The P0420 misdiagnosis that wastes money
P0420 is the code garages love to throw a £700 catalytic converter at — and half the time they're wrong. Bob's 32 years on the tools rule: before you authorise a new cat, DEMAND three tests written down on the invoice. One, a proper smoke-test of the exhaust from manifold to the pre-cat lambda boss — even a pinhole leak before the cat lets fresh air fool the rear O2 and stores P0420 with a perfectly healthy catalyst. Two, a live-data lambda graph on a scan tool showing FRONT and REAR oxygen sensor voltages during a 2,500 RPM held-throttle: a healthy cat makes the rear sensor sit dead flat around 0.65V while the front swings 0.1V–0.9V ten times a second; if the rear is mirroring the front the cat is genuinely tired, but if the rear is just LAZY (slow to respond, sits around 0.4V) the rear sensor itself is worn — £120 fitted, not £700. Three, resolve any misfire or fuel-trim code (P0300-series, P0171, P0174) BEFORE touching the cat: raw fuel dumped into a cat from a misfiring cylinder is what killed the last one, and it'll kill the replacement in six months. The other classic miss is fitting a cheap eBay 'universal' cat to a Toyota or Honda hybrid — the precious-metal loading is a fraction of OE, P0420 stores again inside 500 miles and the car fails its next MOT.
What a UK garage should charge for P0420
UK independent garage: diagnostic + full lambda live-data test £60–£90. Rear (post-cat) oxygen sensor replacement £110–£180 fitted — this fixes P0420 on around 35–45% of cars we see, especially Vauxhall A14NET, BMW N-series and older Mercedes. Exhaust leak repair (welded patch or new flexi) £45–£140. Front and rear lambda pair £220–£380 fitted. Type-approved aftermarket cat (BM Cats, Klarius, Bosal — with UK MOT paperwork) £320–£620 fitted. Genuine OEM catalytic converter £700–£1,600 fitted (Toyota hybrid OE cats commonly £900–£1,300 alone). Cat + heat shield theft repair on a Toyota Yaris hybrid £900–£1,800 including a Catloc / Armaloc shield. Never accept a quote for cat replacement without a written lambda graph — that paperwork is your evidence if the code returns within 90 days.
P0420 repair cost in the UK
Most P0420 repairs land between £180–£1400 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on P0420
Three things Bob would do before spending a penny on a new cat. First, get the rear lambda replaced with a genuine or Denso/NGK part (never a cheap eBay lambda — they read lazy from day one and store P0420 again). Second, drive the car HARD once a week — thirty minutes of dual-carriageway at 3,500+ RPM burns off carbon and can genuinely regenerate a marginal cat back to a passing lambda swing; short-trip city cars kill cats fast, and a proper Italian tune-up has cleared MOT-fail P0420 more than once in the workshop. Third, if you're on a Toyota, Lexus, Honda or Mitsubishi hybrid — fit a Catloc, Armaloc, or Toyota's own hybrid catshield BEFORE the cat gets stolen. Insurers now class Yaris and Jazz hybrid cats as a theft category on par with keyless-entry car theft. If the P0420 arrived alongside a sudden loud exhaust roar, don't waste money on diagnostics — the cat is gone, ring your insurer and get a shield fitted with the new one. And if you drive a Peugeot / Citroën 1.2 PureTech, ring the dealer with your VIN before paying for anything: there is a live PSA software update that clears certain P0420 fault families under campaign at no charge to the owner.
Can you drive with P0420?
Yes, short term — but you'll fail MOT emissions and fuel economy may drop.
Urgency: medium.