2019 Mercedes-Benz P0420: What It Means & Is It Safe to Drive?
P0420 on the 2019 Mercedes-Benz: 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 2019 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 2019 Mercedes-Benz, repairs for P0420 typically fall between $400 and $2400, depending on the root cause and labor rates in your area.
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