P0C21: Electric/Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low
What to do
As P0C05's argument — all three phases means the connector or the driver, not three failures. Symptoms show up pulling away from rest rather than on the move, per P0B00. Ask whether this pump is high or low voltage on this model before any inspection.
What this code means
All three phase circuits driving the auxiliary transmission fluid pump's motor are reading low.
The all-three-at-once argument from P0C05 applies here at a smaller scale and with the same conclusion: three windings do not fail together, so the fault is what they share — the pump's connector, the short harness to it, or its driver stage. The per-phase codes for this pump are at P0B00–P0B08, and the difference is exactly that: those name one phase, this names the set.
Unlike the traction motor, this pump's supply may be low voltage on some designs — establish which before anyone touches it, and treat it as high voltage until you have.
Likely causes
Start here
- Corroded or loose connector · easy
- Damaged transmission wiring harness · moderate
- Faulty auxiliary transmission fluid pump · specialist
Less common
- Open circuit in pump wiring · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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