Open circuit or high resistance in the phase V wiring or connector between the pump controller and the pump motor (corrosion, chafing, loose terminal); Auxiliary pump motor with an open or high-resistance phase V winding; Fault in the pump motor driver or controller (internal output stage failure); Water or fluid ingress into the pump connector or controller causing poor contact; Low supply voltage or poor power and earth connections to the pump controller affecting current output; Outdated or faulty control unit software, only where the manufacturer has documented it as a cause for this code.

P0B04

Electric/Auxiliary Transmission Fluid Pump Motor Phase V Current Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
B: AreaHybrid propulsion
04: FaultThe specific fault in that area

What it means

P0B04 means the control unit that drives the electric auxiliary transmission fluid pump saw the current in the motor's phase V winding lower than expected while commanding the pump. This points to an open or high-resistance circuit, a motor fault or a driver fault. Layout and strategy vary by model and transmission, so check the manufacturer's data for your vehicle.

Symptoms

Warning light on, possibly with a transmission or hybrid system message
Delayed or harsh engagement of drive or reverse, especially after stop-start or when pulling away
Transmission limp mode or restricted gear selection
Engine stop-start disabled or a hybrid system running in a reduced mode
Whining, buzzing or no noise from the auxiliary pump when it should be running
Transmission overheating or slipping under load

Possible causes

Open circuit or high resistance in the phase V wiring or connector between the pump controller and the pump motor (corrosion, chafing, loose terminal)likely
Auxiliary pump motor with an open or high-resistance phase V windinglikely
Fault in the pump motor driver or controller (internal output stage failure)possible
Water or fluid ingress into the pump connector or controller causing poor contactpossible
Low supply voltage or poor power and earth connections to the pump controller affecting current outputpossible
Outdated or faulty control unit software, only where the manufacturer has documented it as a cause for this coderare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting any related pump, transmission or battery codes
Check transmission fluid level and condition, and look for leaks around the pump, using the correct procedure for the model
Visually inspect the pump, controller and wiring for damage, chafing, heat marks, corrosion and water ingress, and check connectors are fully seated
Check battery and supply voltage, and the power and earth feeds to the pump controller
Use a scan tool that supports live data to command the pump on and compare the commanded and actual pump speed and current, if available
With the circuit isolated and following the manufacturer's safety procedure (especially if high-voltage), test phase V wiring continuity and the motor winding resistance against the three phases and the specification
Check for software updates or technical bulletins for the model and fault code, and only then consider the controller or pump as faulty

How to fix

Repair or replace damaged wiring and clean or replace corroded or loose connector terminals in the phase V circuit
Replace the auxiliary transmission fluid pump motor if the winding is open or out of specification
Replace or repair the pump controller if the driver output is faulty and the wiring and motor test good
Correct power or earth supply faults, and dry out and seal any area where water is entering
Apply a manufacturer-approved software update if an outdated calibration is a documented cause, then clear the code and road test to confirm it does not return

Can I keep driving?

Medium severity

Short, gentle driving may be possible, but without the auxiliary pump the transmission may lack pressure or cooling and can be damaged. Have it checked soon, and stop driving if it slips, overheats or refuses to engage a gear.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.