What Does Code P0420 Mean on a 2024 Mercedes-Benz C-Class?
A P0420 code on your 2024 Mercedes-Benz C-Class (W206) indicates the engine control unit (ECU) has detected reduced Bank 1 catalytic converter efficiency. The 2024 C-Class features the C300 with a 2.0L M254 turbocharged inline-4 engine producing 255 horsepower and 295 lb-ft of torque, equipped with a 48V mild hybrid system (EQ Boost). The AMG C43 uses a 2.0L M139 turbo-4 with an electric turbocharger making 402 hp. Both use the 9G-TRONIC automatic.
On a brand-new Mercedes, P0420 is unusual and almost certainly a warranty-covered defect.
Common Symptoms
- Check engine light (yellow engine icon) illuminated
- Possible "Visit Workshop" message on MBUX
- Slight fuel economy decrease
- Emissions test failure
- No significant performance reduction
Most Likely Causes on the 2024 C-Class
1. ECU Software Calibration
The M254 engine with EQ Boost mild hybrid is a sophisticated powerplant. Software calibration updates for catalyst monitoring are the most likely fix for a new vehicle.
2. Faulty Lambda Sensor
A manufacturing defect in the downstream oxygen sensor can produce false readings. Mercedes uses high-precision wideband sensors that are sensitive to even minor defects.
3. Exhaust Assembly Defect
A factory assembly issue such as a loose V-band clamp or improperly seated gasket can create a leak.
4. Catalytic Converter Defect
Rare on a new vehicle. The close-coupled converter design on the M254 positions it near the turbo for quick light-off.
How to Diagnose the P0420 Code
- Take the vehicle to a Mercedes-Benz dealer for warranty service
- Mercedes XENTRY/DAS diagnostic system will identify specific faults
- ECU software version check and update
- Lambda sensor and exhaust system analysis
- Converter efficiency testing per Mercedes procedures
Repair Options and Costs
Warranty Repair (Recommended)
- All P0420-related repairs: No cost under Mercedes warranty
Out-of-Warranty Reference (Mercedes Pricing)
- Catalytic converter (C300 M254): $1,800–$3,500
- Catalytic converter (AMG C43): $2,500–$4,500
- Lambda sensor: $280–$600
- Software update: $200–$400
DIY Considerations
Do not attempt DIY repairs on a 2024 Mercedes-Benz under warranty. Mercedes requires authorized service for warranty coverage.
Cost Breakdown Summary
- Under warranty: $0
- OEM converter (C300): $1,200–$2,800
- OEM converter (AMG C43): $1,800–$3,500
- Lambda sensor: $180–$450
Prevention Tips
- Follow Mercedes-Benz Service A and Service B maintenance intervals
- Use Mercedes-approved MB 229.71 or 229.72 synthetic oil
- Use premium 91+ octane fuel (93 recommended)
- Keep ECU software current through dealer visits
- Do not install aftermarket exhaust components that could void warranty