What Does P0121 Mean on a 2024 Mercedes-Benz C-Class?
A P0121 code on your 2024 Mercedes-Benz C-Class (W206) indicates the ECU has detected an abnormal voltage pattern from the throttle position sensor (TPS). The signal is present and within range, but doesn't behave as the engine management system expects for your driving conditions. On a new Mercedes, this warrants immediate dealer investigation.
The 2024 C-Class features the M254 2.0L turbocharged four-cylinder with EQ Boost mild-hybrid assistance across the lineup, or the M139 2.0L turbo-four in the AMG C 43. Both use direct fuel injection. Mercedes-Benz's engine management system integrates the TPS into the electronic throttle body. The mild-hybrid system's 48V integrated starter-generator (ISG) adds an additional layer of powertrain complexity. At low mileage, software is the primary suspect.
Top Causes of P0121 on the 2024 C-Class
- ECU software calibration – New platform software may need throttle mapping updates. Likelihood: 40%
- Factory throttle body defect – Manufacturing issue with the integrated TPS. Likelihood: 25%
- Connector not properly seated – Assembly oversight. Likelihood: 20%
- 48V mild-hybrid system interaction – ISG torque fill during gear changes may affect throttle correlation. Likelihood: 10%
- Carbon buildup – Unlikely at low mileage. Likelihood: 5%
Symptoms You May Notice
- Check Engine Light on (yellow engine icon on MBUX)
- Rough or inconsistent idle
- Hesitation on acceleration
- EQ Boost transitions may feel rough
- Reduced engine power message
- Cruise control inoperative
How Serious Is This Code?
P0121 is medium severity. Your 2024 C-Class remains drivable, but any engine management code on a new Mercedes should be addressed immediately. Schedule a dealer visit soon—within the week. Full warranty coverage applies.
Diagnostic Steps
- Take the vehicle to your Mercedes-Benz dealer—warranty applies
- Dealer uses Mercedes XENTRY/DAS for full diagnostic analysis
- Check for software updates and SCN coding updates
- Inspect throttle body connector and wiring assembly quality
- Monitor live TPS data alongside 48V system parameters
- Replace throttle body if hardware defect confirmed
Estimated Repair Costs
- Under warranty: $0 (covered by Mercedes 4-year/50,000-mile warranty)
- ECU software update: $0 under warranty
- Throttle body replacement (if out of warranty): $550–$1,000
Should You DIY This?
DIY feasibility is low. Your 2024 C-Class is fully covered under Mercedes-Benz's 4-year/50,000-mile bumper-to-bumper warranty. Let the dealer handle everything. Mercedes XENTRY diagnostic system is required for proper coding and software updates. DIY work could jeopardize warranty coverage.