Damaged or corroded wiring harness to the auxiliary pump, especially at connectors or where the harness flexes; Failed auxiliary transmission fluid pump motor (shorted or open phase winding); Poor ground connection for the pump or TCM; Faulty transmission control module (TCM) driver circuit for the pump; Low transmission fluid level or contaminated fluid causing pump overheating and phase imbalance; Internal short in the pump connector or terminal back-out

DTC codes / P0Cxx · Hybrid propulsion / P0C21

P0C21Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low

P0C21 means the transmission control module (TCM) has detected a low-voltage condition on the phase U, V, or W circuit of the electric auxiliary transmission fluid pump. The pump is a three-phase brushless motor used to maintain fluid pressure when the engine is stopped, and the TCM monitors each phase for abnormal voltage drop or current draw.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaHybrid propulsion

Symptoms you may notice

Illumination of the check engine light or transmission warning light, often with no other symptoms
Harsh or delayed shifting, especially when the engine restarts after an auto-stop event
The vehicle may enter limp mode or refuse to engage drive/reverse after an idle-stop
Extended crank or no-start condition if the auxiliary pump is needed for initial pressure build
Intermittent loss of transmission engagement while driving at low speeds

Common causes, most likely first

Damaged or corroded wiring harness to the auxiliary pump, especially at connectors or where the harness flexes
Failed auxiliary transmission fluid pump motor (shorted or open phase winding)
Poor ground connection for the pump or TCM
Faulty transmission control module (TCM) driver circuit for the pump
Low transmission fluid level or contaminated fluid causing pump overheating and phase imbalance
Internal short in the pump connector or terminal back-out

How to diagnose and fix it

Check transmission fluid level and condition; top off or replace fluid if contaminated
Inspect the pump wiring harness and connectors for chafing, corrosion, or loose terminals
Measure resistance across each phase of the pump motor and compare to specifications
Check for voltage drop on the pump power and ground circuits under load
Test the TCM output signals to the pump with an oscilloscope or bidirectional scan tool
Replace the auxiliary transmission fluid pump if electrical tests confirm an internal fault
Clear codes and perform a relearn or adaptation drive cycle to verify the repair

Auxiliary pump faults that fool technicians

A low fluid level or a clogged pump pickup can cause phase current imbalance that sets P0C21 without any wiring fault.
The pump may test fine statically but fail only when hot or under load, so a road test with live data is essential.
Corrosion inside the pump connector is a common hidden cause that visual inspection alone can miss.
A failing TCM can command the pump incorrectly, so always rule out the controller before replacing the pump.
This code can be stored alongside misfire or idle-stop codes because the pump supports engine restart smoothness.

Can I keep driving with P0C21?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy loads, prolonged idling, and relying on the auto-start-stop feature. If the transmission slips or refuses to engage, stop driving and have it towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.