What does code P0420 mean on a Mercedes-Benz and is it safe to drive?
P0420: Catalyst System Efficiency Below Threshold (Bank 1). Here's what it means on your Mercedes-Benz, what usually causes it, what repairs typically cost, and how safe it is 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 for your Mercedes-Benz
- 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 Mercedes-Benz, repairs for P0420 often fall between $400 and $2400, depending on the cause and whether you use OEM or aftermarket parts. Labor rates vary by location.
Known issues on Mercedes-Benz
Mercedes P0420 is frequently caused by secondary air injection (SAI) system faults rather than cat failure—the SAI pump injects air during cold starts; when it fails, the engine runs lean at startup and the ECU misinterprets catalyst efficiency. Always diagnose the SAI system before replacing the converter on M271, M272, M274 engines. On C300 2.0T (M274), the catalyst is integrated with the downpipe as a single assembly (OEM $1,200–$2,500). Check for active oil consumption on M271 1.8T engines, which contaminate the catalyst.
Most commonly affected models