One of the most common emissions-related fault codes on BMW petrol engines. This code indicates the catalytic converter isn't cleaning exhaust gases as effectively as it should – but the root cause is often not the catalyst itself. Here's what to check and in what order.
P0420 means the rear oxygen sensor is reporting that the catalytic converter isn't reducing emissions as efficiently as the ECU expects – typically a 30%+ drop in conversion efficiency. The most common cause is not a failed catalyst, but a faulty oxygen sensor, an exhaust leak, or an engine misfire contaminating the catalyst.
Do not replace the catalytic converter until the oxygen sensors and exhaust system have been properly checked. Full diagnostic order below.
In a modern BMW petrol engine, the catalytic converter is monitored by two oxygen sensors – one before the catalyst (pre-cat) and one after (post-cat). The ECU compares the signals; the pre-cat sensor fluctuates rapidly, while the post-cat sensor should be relatively stable. P0420 is set when the post-cat sensor's readings start to mirror the pre-cat sensor, indicating the catalyst isn't storing oxygen efficiently – typically a drop of 30% or more in conversion efficiency. Crucially, the code describes a measurement, not a diagnosis. The catalyst could genuinely be failing, but it's far more common for a faulty oxygen sensor, an exhaust leak, or an engine management issue to cause the same reading. This code appears across all BMW petrol families – from the N20 four-cylinder turbo to the N55 and B58 six-cylinder units – and the diagnostic approach is largely the same.
Ordered by how often each turns out to be the real culprit – with typical fitted cost against each, so you can sanity‑check any quote you've been given.
| Cause | Likelihood | Typical Cost (fitted) | Notes |
|---|---|---|---|
| Faulty rear oxygen sensor (post-cat) | Very High | £200–£400 | Reports incorrect readings; often the cheapest fix |
| Exhaust leak before the catalytic converter | High | £100–£400 | Allows fresh air into the exhaust, confusing the sensors |
| Engine misfire (unburned fuel damaging catalyst) | Medium | £300–£1,200+ | Check for P0300 codes first; may need catalyst later |
| Oil or coolant contamination of the catalyst | Medium | £800–£1,500 | Internal engine issues (e.g. head gasket) can destroy the catalyst |
| Catalytic converter failure (genuine) | Low | £800–£1,500 | Rule out everything above before replacing |
Cost figures [PATTERN DATA] – indicative independent BMW specialist pricing, not drawn from our engine replacement dataset.
Inspect the exhaust system between the engine and the catalytic converter. Even a small leak can cause P0420.
Compare the post-cat sensor readings with the pre-cat sensor. If they're almost identical, suspect the sensor or the catalyst.
If P0300 or individual cylinder misfire codes are present, fix those first – they may have damaged the catalyst.
Watch the post-cat sensor readings over a full drive cycle – a slow-responding sensor is often the issue.
If everything else is clear and the catalyst truly isn't working, replacement is the only fix.
Our diagnostic wizard walks the same order, or confirm your engine with the registration lookup.
Yes, but with caution. P0420 itself is not immediately engine‑destroying. However, if the code is caused by a misfire, continued driving can further damage the catalytic converter, turning a £300 repair into a £1,500 one. If the car has misfire codes or runs rough, avoid driving until diagnosed. If the car runs normally but has the light on, you can drive it for short periods, but get it diagnosed within a few weeks to avoid MOT failure.
Yes, for short trips. Avoid driving if the engine is misfiring or running rough – that can damage the catalytic converter further. If the car runs normally, you can drive it for a short period, but get it diagnosed before your next MOT.
No. Often it's a faulty oxygen sensor or an exhaust leak – both far cheaper than a new catalyst. Always check those first.
Only temporarily. The code will return as soon as the ECU runs the catalyst monitor again – usually within one drive cycle. Clearing it erases freeze‑frame data that helps with diagnosis.
Expect 1–2 hours of diagnostic time at an independent BMW specialist, typically £100–£200. Many specialists will waive or offset this if you proceed with the repair.
Some BMW models had emissions-related recalls or extended warranties on catalytic converters (e.g., certain N20 engines). Check your VIN against BMW UK recalls or use our recall checker.
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