P28C4
Clutch A Phase U-V-W Circuit Low
The output stage for the clutch A BLDC motor reports a low phase voltage on at least one of the U/V/W phases. Short to ground or interrupted half-bridge.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: hard
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
Common causes
Ordered by how often each turns out to be the cause.
- Phase wire shorted to groundCommon
- Low-side switch open in driverPossible
- Connector pins corroded - high contact resistanceLess likely
Parts involved
- Clutch Motor A
- Wiring Harness
- TCM
P28C4 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.