What Does P0121 Mean on a 2024 GMC Sierra?
A P0121 code on your 2024 GMC Sierra indicates the ECM has detected an abnormal TPS voltage pattern. The signal is present but doesn't match expected behavior. On a new Sierra, this should be addressed under warranty.
The 2024 Sierra offers the 2.7L turbo-four (direct injection), 5.3L V8 (direct + port injection), or 6.2L V8 (direct + port injection). GM's Dynamic Fuel Management (DFM) on V8 models depends heavily on precise throttle data. The electronic throttle body integrates the TPS. At low mileage, software calibration is the primary suspect.
Top Causes of P0121 on the 2024 Sierra
- ECM software calibration – GM regularly releases throttle updates. Likelihood: 40%
- Factory throttle body defect – Manufacturing issue. Likelihood: 25%
- Connector assembly issue – Not properly seated. Likelihood: 20%
- DFM system interaction – Cylinder deactivation affecting throttle data. Likelihood: 10%
- Carbon buildup – Unlikely at low mileage. Likelihood: 5%
Symptoms You May Notice
- Check Engine Light on
- Rough or inconsistent idle
- Hesitation during acceleration
- Reduced towing power
- DFM may not engage (V8)
- Cruise control issues
How Serious Is This Code?
P0121 is medium severity. Your 2024 Sierra remains drivable. Schedule a dealer visit soon—within the week. Full warranty applies.
Diagnostic Steps
- Take the Sierra to your GMC dealer—warranty covers this
- Dealer uses GDS2 for GM-specific diagnostics
- Check for software updates and TSBs
- Inspect throttle body and connector
- Monitor TPS alongside DFM data (V8)
Estimated Repair Costs
- Under warranty: $0 (GM 3-year/36,000-mile basic + 5-year/60,000-mile powertrain)
- ECM reflash: $0 under warranty
- Throttle body replacement (if out of warranty): $350–$650
Should You DIY This?
DIY feasibility is low. Your 2024 Sierra is under full GM warranty. Let the dealer handle it at no cost. GDS2 is required for proper diagnostics and software updates.