What it means
The output stage for the clutch B BLDC motor reports a low phase voltage on at least one of the U/V/W phases. Short to ground or interrupted half-bridge.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
The clutch B brushless motor is driven through three output phases, U, V, and W, and the TCM monitors their voltage at the output stage. P28D2 sets when at least one clutch B phase voltage is too low, which can result from a phase wire short to ground, an open low-side driver switch, or a poor connector contact.
Common causes
Most likely first.
- Phase wire shorted to ground
- Low-side switch open in driver
- Connector pins corroded - high contact resistance
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
How to diagnose it
- Save freeze-frame data and review live transmission data for clutch B motor phase status or driver faults when the code sets.
- With the ignition off, inspect the clutch B motor phase connector and harness for corrosion, loose pins, damaged insulation, or contact with ground.
- Use the manufacturer's wiring diagram to identify the U, V, and W phase circuits and check each for a short to ground.
- Check continuity and connector contact quality in all three clutch B phase circuits using the manufacturer's procedure.
- If the phase wiring is sound, test the clutch B motor driver low-side output stage as specified by the manufacturer.
Common fixes
- Repair a clutch B U, V, or W phase wire that is shorted to ground.
- Clean or repair corroded clutch B motor connector terminals.
- Restore an open or damaged phase circuit.
- Repair or replace the clutch B motor driver if testing confirms a failed low-side switch.
Can I keep driving?
Avoid driving if gear changes are slipping, harsh, or delayed, because clutch B may not be controlled reliably and the transmission can enter limp mode. Stop if the vehicle loses drive or becomes stuck in a gear.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.