P0B01

What it means

P0B01 means the control unit monitoring the electric or auxiliary transmission fluid pump motor measured less current in motor phase U than it expected while the pump was commanded on. The pump is usually a three-phase motor that keeps fluid pressure up when the engine is stopped, as in stop-start and hybrid systems. Low current on one phase points to an open circuit, high resistance or a fault in the motor or its driver. Layout and strategy vary by model and transmission.

Symptoms

• Warning light or transmission fault message on the dashboard
• Stop-start function disabled or not working
• Delayed or harsh engagement when pulling away after a stop
• Rough or flared shifts, or transmission limp mode in some cases
• Whining or no sound from the auxiliary pump when it should run
• Other transmission or hybrid system codes stored alongside

Possible causes

• Wiring or connector fault in phase U between the pump motor and its driver (open circuit, corrosion, loose pin, high resistance)likely
• Internal fault in the pump motor, such as an open or damaged phase U windinglikely
• Fault in the pump driver or control module output stage for phase Upossible
• Low or unstable system voltage from a weak battery or poor power and earth connectionspossible
• Low or contaminated transmission fluid or a seized or restricted pump causing abnormal currentpossible
• Outdated control software where the manufacturer has identified it as a causerare
• Water ingress or physical damage to the harness near the transmissionrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting voltage, temperature and whether the engine was stopped or running.
2. Check for related codes covering the other motor phases, pump circuit, battery voltage or communication faults, and deal with those first.
3. Check battery condition and charging voltage, and inspect the main power and earth connections.
4. Visually inspect the pump harness and connectors for chafing, heat damage, corrosion, water and pins that are loose or pushed back.
5. Check the transmission fluid level and condition using the correct procedure for the model.
6. Using live data, command the pump if the system allows it and compare the commanded and actual pump speed and current.
7. With the circuit isolated, test phase U continuity and resistance against the manufacturer's figures and compare it with the other phases.
8. Check for manufacturer service bulletins or software updates relevant to the vehicle and fault.

How to fix

• Repair or replace damaged wiring and terminals in phase U, and clean and secure the connectors.
• Replace the pump motor, or the pump assembly if the motor is not supplied separately, if the windings or operation are faulty.
• Replace or repair the pump driver or control module only after the wiring and pump are proven good.
• Restore correct battery condition and repair power or earth faults.
• Correct the fluid level, or service the fluid as the manufacturer specifies, if low or contaminated fluid is found.
• Apply a manufacturer software update only where it is a documented fix for this fault.

Can I keep driving?

Nearby codes

• P0B00GenericElectric/Auxiliary Transmission Fluid Pump Motor Phase U Current
• P0B02GenericElectric/Auxiliary Transmission Fluid Pump Motor Phase U Current High