P0B01: Auxiliary transmission-pump phase U current is too low

P0B01 identifies electric/auxiliary transmission fluid pump motor phase U current low. The 2018 GM HPC1 monitor checks sensed U-phase current for an open-circuit response at the part of the electrical cycle where current should approach its peak.

A zero crossing is not the fault criterion

The listed threshold is peak phase-axis current below 1 A for 0.4 seconds within a 0.6-second window. The drive must be in Run, the inverter in normal PWM rather than active discharge, and inverter voltage above 50 V. The phase-axis evaluation is restricted to a ±30-degree window, with squared commanded current above 14.97 A².

Those enable conditions matter because low instantaneous current can be expected outside the intended phase window or when little current is requested. The code's finding is insufficient current when the controller expects it, not simply any low reading. GM lists Type A, one trip.

What to preserve

Record the U-phase identity, drive state, current request and relevant inverter status. This code does not directly measure transmission-fluid flow or identify a mechanical pump obstruction. It also does not prove a specific conductor is open without the appropriate diagnostic checks.

Use the vehicle-specific motor/inverter procedure and qualified high-voltage service. Verification should establish acceptable phase-current response under an eligible test. Do not directly energize the pump or probe active motor phases from this monitor description.

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 1379.