What it means
The control module detects phase U current below the expected level in the electric/auxiliary transmission fluid pump motor.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Phase U is one of the three electrical phases that drive the electric or auxiliary transmission-fluid pump. P0B01 sets when the control module detects phase-U current below the expected level, commonly because an open or high-resistance connection prevents the motor from drawing enough current.
Common causes
Most likely first.
- Open or high-resistance phase wiring
- Pump motor fault
- Current sensing or control fault
Symptoms
- Transmission or hybrid system warning light
- Transmission operation may be limited
How to diagnose it
- Check freeze-frame data and related pump or transmission codes to see when phase-U current fell below expectation.
- Compare live phase-U current with phases V and W under the manufacturer's specified operating conditions to confirm that the low reading is phase-specific.
- Inspect the pump connector and phase-U wire for an open, loose terminal, corrosion, or heat damage.
- Test phase-U wiring continuity and voltage drop using the wiring diagram and the manufacturer's specifications.
- If the phase-U circuit passes, test the pump motor and current-sensing or control circuit to locate the source of the low reading.
Common fixes
- Reconnect or clean a poor pump connector and secure the wiring.
- Repair an open or high-resistance section of phase-U wiring.
- Replace the pump motor if it fails the manufacturer's phase-current tests.
- Repair the current-sensing or control circuit if the low reading is not caused by wiring or the motor.
Can I keep driving?
Limit driving and avoid heavy loads if the transmission or hybrid system warns of restricted operation; low phase-U current can mean the fluid pump is not circulating enough fluid. Stop if shifting becomes abnormal or the vehicle enters limp mode.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.