P0C20 means at least one of the three motor phases of the auxiliary transmission fluid pump carries no current, typically an open lead, terminal or winding. GM uses it in the pump controller of the Chevrolet Volt (2011-15 tables) and the 2016 Cadillac ELR: with the inverter in Run, high voltage above 35 V and the rotor within 30 degrees of a phase axis, the peak current of that phase is checked; below 1 A for 200 counts sets the code, Type A, MIL on the first trip. Each phase is tested in turn as the rotor turns. The pump cannot build torque, so pressure support and electric operation are lost. Later GM tables split the check into the per-phase codes P0B01, P0B04 and P0B07; P0C21/P0C22 are the low and high variants.
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 Service Vehicle Soon message
- Reduced propulsion or no electric drive mode
- Engine restart harsh or delayed
Most likely causes
- Motor connector of the pump not fully seated, or loose phase terminal
- Open phase lead between pump inverter and motor
- Power switch of one phase in the pump inverter not conducting
- Open winding in the pump motor
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 P0C20 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- C: Subsystem depends on the code range
- 20: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0C20?
Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
What causes code P0C20?
The most common causes are: motor connector of the pump not fully seated, or loose phase terminal; open phase lead between pump inverter and motor; power switch of one phase in the pump inverter not conducting.
Related codes
- P0C21Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low
- P0C22Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit 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
- P0C28Electric/Auxiliary Transmission Fluid Pump A Motor Current High
Sources
- gsi.ext.gm.com: 12OBDG01A HYBRID Diagnostics
- gsi.ext.gm.com: 16OBDG01 Hybrid Diagnostics
- gsi.ext.gm.com: 11OBDG01 HYBRID Diagnostics
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.