P0C20: Pump phase-current absence depends on command and rotor angle

P0C20 appears in GM's auxiliary transmission-fluid-pump controller section as a phase U-V-W circuit/open diagnostic. The monitor checks individual phases as the rotor turns. Its finding is missing expected phase current under qualifying conditions, not merely a low current value at any time.

The command and angle make the reading meaningful

The absolute peak phase-axis current must be below 1 amp while commanded current is at least 3 amps. The inverter must be in RUN, high-voltage supply above 35 volts, and rotor position within 30 degrees of the relevant phase axis. A reading obtained with no current demand or at another rotor position does not reproduce those conditions.

GM lists 200 qualifying counts at a 0.11-millisecond evaluation interval, totaling 22 milliseconds. This is a rapid controller-level assessment; it should not be equated with a slowly updating scan-tool current display.

Keep the controller identity straight

The strategy text calls the mechanism “Drive Motor A” even though the component heading and section identify the auxiliary pump. That source-label inconsistency is retained as an unresolved detail; the article does not redirect the code to a traction-motor replacement.

Use the exact pump/controller service documentation to establish the measurement path and appropriate test. The table does not identify a particular winding, connector or replacement assembly as the cause. High-voltage motor and inverter testing requires qualified personnel, rather than improvised probing based on this monitor summary.

Source and applicability

15OBDG01 Hybrid Diagnostics, cited controller and monitor only. Historical calibration limits are not universal specifications; per-code vehicle, engine and hardware applicability must be verified.

GM source, physical page 699.