Powertrain, generic · Hybrid propulsion

P0C21 Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

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

Symptoms

  • MIL and transmission or hybrid warning
  • Auxiliary pump off, no engine auto-stop

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–2,500
Time1–6 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

aux trans pump phase uvwaux trans pump inverter

Related codes

P0C20P0C22P0C28P0B01P0B04P0B07

References: law.cornell.edu, gsi.ext.gm.com, patents.google.com.