P0B04: Auxiliary transmission-pump phase V lacks expected current

P0B04 concerns electric/auxiliary transmission fluid pump motor phase V current low. GM's 2018 HPC1 table evaluates the sensed V-phase current near its expected electrical peak to detect an open-circuit response.

The current request changes the interpretation

The monitor requires peak phase-axis current below 1 A for 0.4 seconds in a 0.6-second window. It runs with drive state Run, normal PWM operation, inverter voltage above 50 V and active discharge excluded. The evaluation uses a ±30-degree phase-axis window and squared commanded current above 14.97 A².

The command value is in squared amperes; it should not be quoted as a 14.97 A current requirement. More importantly, a low value when the pump is not being actively driven does not reproduce the documented fault. The table gives Type A, one-trip reporting.

Keep phase and system identities separate

V identifies this pump motor's phase, not a traction-motor phase or a hydraulic pressure sensor. Retain the reporting module and requested drive condition before investigating a generic “transmission pump” label. The code itself does not establish pump-flow deficiency or choose a replacement motor.

The applicable high-voltage motor/inverter diagnostic must distinguish the current-sensing and power-path causes. Repair confirmation requires an eligible V-phase response test. It is not safe or sufficient to energize the phase externally or infer success merely from the pump making noise.

Source and applicability

This explanation follows the documented GM monitor, not a verified application to every vehicle with this code. Confirm the vehicle, reporting module and calibration in current service information. GM diagnostic table, physical page 1380.