P0C21 code meaning
Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low
Powertrain system Generic code P0Cxx codes Check engine light
What P0C21 means
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.
P0C21 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 6 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0C21
- MIL and transmission or hybrid warning
- Auxiliary pump off, no engine auto-stop
Common causes of P0C21
- Most 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
- Less likelyMotor winding shorted to the stator core
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Aux trans pump phase uvwAux trans pump inverter
What each part of P0C21 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| C | Area: hybrid propulsion. |
| 21 | The exact fault in that area. |
What to do next
- Note P0C21 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0C21
What does P0C21 mean?
P0C21 means "Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P0C21?
Drive gently and only to a workshop. With P0C21 the car can go into limp mode (reduced power) to protect itself.
Will P0C21 fail an emissions test?
Yes, it can. P0C21 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0C21 to fix?
Difficulty: hard. A typical repair takes 1 to 6 hours of work. This is a job for a workshop.
Will P0C21 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.