What Does P0121 Mean on a 2024 Mercedes-Benz GLC?
A P0121 code on your 2024 Mercedes-Benz GLC (X254) indicates the ECU has detected an abnormal TPS signal pattern. The voltage is present but behaves erratically compared to expected parameters. On a new GLC, this warrants immediate dealer investigation.
The 2024 GLC features the M254 2.0L turbo-four with 48V EQ Boost mild-hybrid technology. It uses direct injection and the electronic throttle body integrates the TPS. The second-generation GLC (X254) also offers the AMG GLC 43 with the M139 2.0L turbo. At low mileage, software calibration is the primary suspect.
Top Causes of P0121 on the 2024 GLC
- ECU software calibration – New platform may need throttle mapping updates. Likelihood: 40%
- Factory throttle body defect – Manufacturing issue with the TPS. Likelihood: 25%
- Connector not properly seated – Assembly oversight. Likelihood: 20%
- 48V EQ Boost interaction – ISG transitions affecting throttle data. Likelihood: 10%
- Carbon buildup – Unlikely at low mileage. Likelihood: 5%
Symptoms You May Notice
- Check Engine Light on MBUX
- Inconsistent idle
- Hesitation during acceleration
- EQ Boost transitions feel rough
- Reduced power message
- Cruise control issues
How Serious Is This Code?
P0121 is medium severity. Your 2024 GLC remains drivable, but schedule a dealer visit soon—within the week. Full warranty applies.
Diagnostic Steps
- Take the GLC to your Mercedes dealer—warranty covers this
- Dealer uses XENTRY for full diagnostics
- Check for software updates and SCN coding
- Inspect throttle body connector and wiring
- Monitor live TPS and 48V system data
- Replace throttle body if hardware fault 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–$950
Should You DIY This?
DIY feasibility is low. Your 2024 GLC is fully covered under Mercedes' 4-year/50,000-mile warranty. Let the dealer handle everything at no cost. XENTRY is required for proper diagnostics and software coding.