What P0121 Means for Your 2021 GMC Yukon
A P0121 diagnostic trouble code on your 2021 GMC Yukon indicates the ECM has detected an abnormal signal pattern from the throttle position sensor. The voltage stays within expected boundaries, but the signal trace doesn't correlate properly with accelerator pedal position—our detective's first clue that something in the throttle system needs investigation.
The 2021 Yukon was the first year of GM's new T1 platform, available with the 5.3L V8 (L84), 6.2L V8 (L87), or the 3.0L Duramax diesel (LM2). The gas V8s feature Dynamic Fuel Management (DFM) with 17 cylinder deactivation patterns, and both use a dual-injection system (port and direct injection). The electronic throttle body is critical for managing airflow across these DFM transitions.
Symptoms You May Experience
- Reduced Engine Power warning on the instrument cluster
- Rough transition between DFM cylinder deactivation modes
- Hesitation on acceleration from a stop
- Unstable idle, especially in Park or at traffic lights
- Check engine light illuminated
Most Common Causes
- Carbon buildup on the throttle body – Even with dual injection, the throttle body accumulates deposits over time. DFM's frequent transitions between firing patterns can exacerbate deposit formation. Likelihood: 40%
- Throttle body motor or sensor degradation – The integrated TPS within the electronic throttle body may develop internal wear. Likelihood: 25%
- Wiring harness or connector issue – The full-size SUV's engine bay generates significant heat, and the throttle body connector can develop corrosion or loose pin contact. Likelihood: 15%
- ECM software calibration – The T1 platform received multiple throttle and DFM recalibrations in its first years. An outdated calibration may cause false P0121 triggers. Likelihood: 15%
- Intake manifold air leak – The active fuel management system relies on precise airflow measurement; even a small leak throws off TPS correlation. Likelihood: 5%
What Repairs Will Cost
- Throttle body cleaning: $100–$180 (shop) / $10–$25 (DIY)
- Throttle body replacement: $350–$650 parts and labor
- Wiring/connector repair: $100–$300
- ECM recalibration: $100–$200 (dealer)
Warranty & DIY Guidance
Your 2021 Yukon is within GM's 5-year/60,000-mile powertrain warranty and may still be covered under the 3-year/36,000-mile bumper-to-bumper depending on purchase date and mileage. Check with your GMC dealer—this should be a covered repair. The dealer will use GDS2 to run a throttle body relearn and verify whether a software update resolves the issue before replacing hardware.
How AutoDetective AI Can Help
Feed your diagnostic data into AutoDetective's AI engine for a detailed analysis specific to your Yukon's engine variant and mileage. Our system identifies the most probable cause and estimates repair costs in your zip code.