2018 BMW P0420: What It Means & Is It Safe to Drive?
P0420 on the 2018 BMW: Catalyst System Efficiency Below Threshold (Bank 1). Here's what it means, what usually causes it, what repairs typically cost, and whether it's safe to keep driving.
What this code means
The downstream oxygen sensor detects that the catalytic converter is not storing or converting exhaust properly. This is one of the most common check engine codes.
Common causes on the 2018 BMW
- Failed or aging catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak before or after the catalyst
- Engine misfire or rich/lean condition damaging the catalyst
Symptoms
- Check Engine Light on
- Often no drivability issues
- Possible failed emissions test
- Rotten egg smell in some cases
Typical fixes
- Replace catalytic converter
- Replace oxygen sensor(s)
- Repair exhaust leaks
- Fix underlying fuel/ignition issues first
Repair cost range
For a 2018 BMW, repairs for P0420 typically fall between $400 and $2400, depending on the root cause and labor rates in your area.
Known issues on BMW
BMW P0420 on N52 and N20 four-cylinder engines often follows HPFP (high-pressure fuel pump) failure on the N20—a rich-running event from HPFP issues can destroy the downstream O2 sensor reading and damage the catalyst. On N52 engines, the Valvetronic eccentric shaft position sensor can cause incorrect fuel trim and eventually P0420. BMW catalytic converters are expensive OEM ($800–$1,500); aftermarket equivalents exist but may not pass CARB states. Confirm the O2 sensor adaption values are within spec before condemning the catalyst.
Most commonly affected models