The P0823 code in your 2021 GMC Sierra indicates a malfunction in the gear lever Y position circuit. This refers to the vertical (up-down or forward-back) position sensing of the shift lever.
Understanding Y Position Sensing
The Sierra's shift mechanism uses sensors to detect lever position in multiple planes. The Y position circuit monitors vertical movement between gear positions like Park, Reverse, Neutral, and Drive.
Common Causes of P0823
- Y position sensor failure
- Shifter mechanism wear
- Wiring damage in shifter assembly
- TCM input circuit malfunction
- Connector corrosion
- Mechanical obstruction in shift path
Symptoms of Y Position Circuit Failure
- Vertical gear positions misread
- Shift interlock malfunctions
- Check engine light on
- Transmission doesn't match selection
- Display shows wrong gear
- Starting issues possible
Sierra Shifter Configuration
The Sierra offers column or console shifters depending on configuration. Both use position sensors for accurate gear determination. P0823 specifically indicates the up-down (Y axis) sensing is affected.