P0420 ("catalyst system efficiency below threshold, bank 1") is one of the most common engine management codes on UK petrol cars over 8 years old. It does **not** automatically mean you need a new catalytic converter — in fact, about half of P0420s are caused by something cheaper.
What P0420 actually checks
The ECU compares the readings from two oxygen sensors — one before the cat and one after. If both look too similar, the ECU decides the cat isn't storing oxygen properly and logs P0420.
Common UK causes, cheapest first
1. **Failed downstream (post-cat) oxygen sensor** — £90–£220 fitted 2. **Exhaust leak before the cat** — £80–£250 to weld 3. **Misfire fouling the cat** — fix the misfire first (coils/plugs £80–£200) 4. **Burning oil contaminating the cat** — sort the oil consumption first 5. **Genuinely failed cat** — £450–£1,200 fitted
Will it fail the MOT?
Yes — a stored P0420 with the EML on is an automatic MOT failure under the post-2018 emissions rules. Clearing the code without fixing the underlying fault is illegal (DVSA fraud).
Can I just clear the code? — It will come back within 50–200 miles of driving. The car self-tests the cat on every drive cycle.
Do I need a genuine cat? — No, but use a Euro-approved type-approved aftermarket cat — cheap eBay cats fail again within months and aren't MOT-legal.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email bob@bobsmechanicalrepairs.co.uk.