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.
Medium severity. Can I keep driving?Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
Symptoms
- MIL and transmission or hybrid warning
- Auxiliary pump off, no engine auto-stop
Most likely causes
- Phase lead chafed through and shorted to ground or the transmission case
- Transmission fluid or water in the motor connector creating a leakage path
- Low-side switch or phase sensing in the pump inverter faulty
- Motor winding shorted to the stator core
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It can trigger limp mode.
- It affects emissions.
What each character of P0C21 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- C: Subsystem depends on the code range
- 21: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0C21?
Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
What causes code P0C21?
The most common causes are: phase lead chafed through and shorted to ground or the transmission case; transmission fluid or water in the motor connector creating a leakage path; low-side switch or phase sensing in the pump inverter faulty.
Related codes
- P0C20Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit/Open
- P0C22Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit High
- P0C28Electric/Auxiliary Transmission Fluid Pump A Motor Current High
- P0B01Electric/Auxiliary Transmission Fluid Pump Motor Phase U Current Low
- P0B04Electric/Auxiliary Transmission Fluid Pump Motor Phase V Current Low
- P0B07Electric/Auxiliary Transmission Fluid Pump Motor Phase W Current Low
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.