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DTC codes / P0Bxx · Hybrid propulsion / P0B08

P0B08Electric/Auxiliary Transmission Fluid Pump Motor Phase W Current High

P0B08 indicates that the transmission control module (TCM) has detected excessive current draw on phase W of the electric auxiliary transmission fluid pump motor, which is used to maintain fluid pressure when the engine is stopped or during certain hybrid driving modes. The TCM monitors the pump motor's phase currents and sets this code when the current on phase W exceeds the expected threshold, typically indicating a short to ground or a faulty pump motor.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyProfessional
AreaHybrid propulsion

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) on the dashboard, often with no other noticeable symptoms.
  • The transmission may enter limp mode or fail to engage drive or reverse smoothly, especially after an auto-stop event.
  • Harsh or delayed gear shifts as auxiliary pump pressure is lost during engine-off periods.
  • A whining or humming noise from the transmission area when the ignition is on but the engine is off, indicating the pump is running abnormally.
  • In hybrid vehicles, reduced electric-only driving range or frequent engine restarts due to insufficient transmission fluid pressure.

Common causes, most likely first

  1. Faulty auxiliary transmission fluid pump motor with an internal short on phase W winding.
  2. Damaged or chafed wiring harness between the TCM and the auxiliary pump motor, causing a short to ground on the phase W circuit.
  3. Corroded or loose electrical connector at the auxiliary pump motor or TCM, leading to excessive current draw.
  4. Failed TCM driver circuit for phase W, which may incorrectly command or monitor the pump motor current.
  5. Contaminated transmission fluid or a mechanical binding in the auxiliary pump, causing the motor to draw higher current than normal.
  6. Aftermarket or incorrect auxiliary pump motor installed previously, with mismatched electrical characteristics.

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related TCM or pump motor codes before proceeding.
  2. Inspect the auxiliary pump motor wiring harness and connector for chafing, corrosion, or loose pins, and repair as needed.
  3. Measure the resistance of the phase W winding at the pump motor connector and compare to specifications to identify an open or shorted motor.
  4. Perform a current draw test on the phase W circuit using a low-current probe to confirm excessive current under commanded activation.
  5. Check the transmission fluid level and condition; replace fluid if contaminated or if the pump is mechanically binding.
  6. If the motor or TCM driver is faulty, replace the auxiliary pump motor or TCM as required, following the manufacturer's diagnostic procedure.
  7. Clear the code and perform a road test with multiple auto-stop events to verify the repair and ensure the code does not return.

Don't overlook the pump motor connector

  • A common oversight is replacing the auxiliary pump motor when the real fault is a chafed wire in the harness near the transmission mount, which shorts only when the engine moves under load.
  • The TCM may set P0B08 intermittently if the 12-volt supply to the pump motor is weak due to a failing battery or charging system, so always check system voltage under load.
  • On some vehicles, the auxiliary pump runs only during auto-stop events, so the code may not set during a short test drive; a scan tool with bidirectional control is needed to command the pump on.
  • A failed auxiliary pump motor can also set codes for other phases (U, V) or for pump performance, so P0B08 alone does not always mean the motor is the only problem.
  • After replacing the pump motor or TCM, some vehicles require a relearn or adaptation procedure for the transmission to properly control the auxiliary pump.

Can I keep driving with P0B08?

You can usually drive the vehicle to a repair shop, but avoid prolonged idling with the engine off and expect reduced transmission performance or limp mode during auto-stop events. If the vehicle enters limp mode or the transmission slips, stop driving and seek professional diagnosis.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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