BMW fault codes P0420 and P0430: catalytic converter efficiency below threshold
The rear oxygen sensor is seeing the same oxygen swings as the front one, so the DME concludes the catalyst on bank 1 (P0420) or bank 2 (P0430) is no longer converting.
What a buyer should take from P0420
P0420 is often the last code, not the first: something upstream killed the catalyst. A used BMW with P0420 and a history of misfires, oil consumption (N62 / N63) or coolant leaks has a £1,500 to £2,500 genuine-catalyst bill waiting, and cheap aftermarket cats tend to relight the code. On a modified car it may simply mean the sports cat has no matching software, which is your cue to ask what else has been changed.
Symptoms that go with P0420
- Engine light with no driveability change
- Rotten-egg smell from the exhaust on a failing catalyst
- MOT emissions failure on the lambda test
- Reduced power only when the catalyst is physically blocked (rare)
- Rattle from under the car when the catalyst substrate has broken up
Causes, most likely first
Systems involved: Exhaust catalysts, oxygen sensors, upstream engine health. Costs are indicative UK independent-specialist figures for 2026; main dealers add 30 to 60 per cent.
| Cause | How likely | Typical UK cost |
|---|---|---|
| Catalyst genuinely worn out, usually after years of misfires, oil burning or coolant ingestion upstream Aftermarket cats often fail the DME's efficiency test within a year | Common past 120,000 miles | £400 aftermarket to £1,500 to £2,500 genuine per catalyst |
| Rear (post-cat) oxygen sensor ageing and reporting a lively signal Cheaper to try first if the catalyst is not rattling | Common | £120 to £250 per sensor |
| Exhaust leak between the front sensor and the catalyst letting air in Manifold-to-cat joints and cracked manifolds on N52 / N54 | Common | £50 to £300 |
| Misfire or lean condition damaging the catalyst (see 29CC, 29F1) | The usual root cause | Fix the upstream fault first |
| Decat or sports cat fitted (tuned cars); software not adjusted Illegal for road use in the UK on a car that had cats from new | Common on modified N54 / N55 | £0 to £600 |
How to diagnose it, cheapest first
- Ask whether the exhaust has been modified; look under the car
- Live data: front and rear lambda traces at idle and at 2,500 rpm; a rear sensor that mirrors the front is a dead cat, a rear sensor that is flat and slow is usually fine
- Check for exhaust leaks and for misfire or lean codes stored with it
- Tap the catalyst body: a rattle means the substrate has broken up
Buy, negotiate, or walk away
Fine to buy
Rear lambda sensor or exhaust leak repaired with a receipt and the code has stayed off for a full drive cycle; catalyst not rattling.
Negotiate
Catalyst confirmed dead on a high-mileage car with an otherwise healthy engine: negotiate the catalyst cost.
Walk away
P0420 with ongoing misfires, oil consumption or coolant loss on an N62 / N63, or a decatted car the seller describes as standard.
Seen P0420 on a listing or a scan?
Paste the listing, reg or VIN. Bimmer.AI identifies the exact engine, applies its known failures and costs, and tells you whether this code is a £60 part or a £2,000 conversation on that car.
Run a Bimmer.AI buyer report →Fault-code guidance on this page is general and engine-family based. The same code can have different causes on different engines and years; a proper diagnosis with live data by a BMW specialist is the only way to be sure. Costs are indicative UK ranges that vary by region and parts supply.
Frequently asked questions
Will a BMW fail its MOT with P0420?
The engine light itself is an MOT failure in the UK since 2018 if it is on during the test, regardless of the code. A worn catalyst may also fail the emissions test. So yes, one way or the other, P0420 needs resolving before the MOT.
Can I fit a cheap catalytic converter to a BMW?
You can, but the DME's efficiency monitor is stricter than the MOT test and many budget catalysts relight P0420 within months. A genuine or OE-supplier catalyst is £800 to £2,500 depending on the engine and whether it is integrated with the manifold (N52 / N54 / N55). Fix the upstream cause first or the new one dies the same way.
Is P0420 the sensor or the catalyst?
Usually the catalyst, sometimes the rear sensor. The rear (post-cat) lambda sensor on a healthy system shows a slow, flat voltage; if it swings like the front sensor the catalyst is not storing oxygen and is worn out. If the rear trace is erratic or dead, the sensor is at fault. A five-minute live-data check tells them apart.