P0C21Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P0C21 reports a low condition on the phase U, V or W circuits of the auxiliary pump motor, usually a phase lead shorted to ground or a voltage at a phase terminal that stays below what the inverter commands. No manufacturer documents this exact code publicly. GM watches the same wiring on the Volt and ELR with the open-circuit code P0C20 and catches shorts through overcurrent on P0C28 and its own phase current sensor codes P1E2A to P1E32. The Hitachi idle-stop pump patent shows that the inverter often reconstructs all phase currents from one DC-bus shunt, so a ground short and a sensing fault can look alike. The pump usually shuts down, auto-stop is blocked. P0C20 is the open circuit, P0C22 the high side, P0B01 to P0B08 the per-phase current codes.
- What you may notice
- MIL and transmission or hybrid warning
- Auxiliary pump off, no engine auto-stop
- Likely causes
- LikelyPhase lead chafed through and shorted to ground or the transmission case
- PossibleTransmission fluid or water in the motor connector creating a leakage path
- PossibleLow-side switch or phase sensing in the pump inverter faulty
- RareMotor winding shorted to the stator core
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €100 – €2,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.