P0C21
Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low
The auxiliary transmission fluid pump phase U-V-W output reports a circuit low condition. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
- 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 openPossible
- Connector pins corroded - high contact resistanceLess likely
Parts involved
- Aux Trans Pump Phase Uvw
- Aux Trans Pump Inverter
P0C21 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.