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DTC codes / P0Bxx · Hybrid propulsion / P0B07

P0B07Electric/Auxiliary Transmission Fluid Pump Motor Phase W Current Low

P0B07 indicates that the transmission control module (TCM) has detected a low current condition on the W phase of the electric auxiliary transmission fluid pump motor circuit. This pump maintains fluid pressure for the transmission during engine stop-start events or when the engine is off, and low current on one phase means the motor is not drawing the expected amperage, which can prevent the pump from building pressure.

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

Symptoms you may notice

  • Illumination of the check engine light or service transmission warning
  • Often no symptoms besides the warning light, especially if the pump is only used intermittently
  • Harsh or delayed shifting when the engine restarts after a stop-start event
  • Inability to engage drive or reverse smoothly after an engine auto-stop
  • Transmission may enter limp mode or default to a fixed gear if the pump fault is severe
  • Extended crank or stall after auto-stop if the pump fails to maintain pressure

Common causes, most likely first

  1. Damaged or corroded wiring harness at the auxiliary pump motor connector, particularly the W phase terminal
  2. Open or high-resistance winding in the W phase of the auxiliary pump motor
  3. Failed auxiliary pump motor driver circuit inside the TCM or power control module
  4. Blown fuse or relay supplying power to the auxiliary pump motor circuit
  5. Poor ground connection for the auxiliary pump motor or TCM
  6. Internal short or open in the auxiliary pump motor assembly

How to diagnose and fix it

  1. Inspect the auxiliary pump motor fuse and relay, and replace any blown fuse or faulty relay
  2. Disconnect the pump motor connector and check for corrosion, loose pins, or damaged wires on the W phase circuit
  3. Measure resistance of the W phase winding at the pump motor connector and compare to manufacturer specifications
  4. Check for voltage drop on the power and ground circuits to the auxiliary pump motor while commanding the pump on with a scan tool
  5. Use a scan tool to command the auxiliary pump on and observe current draw on the W phase to confirm low current
  6. If wiring and connections are good, replace the auxiliary pump motor assembly
  7. If the motor and wiring test good, diagnose or replace the TCM or power control module as the driver circuit may be faulty

Don't overlook the pump motor connector

  • A common oversight is assuming the pump motor is failed when the real problem is a corroded or backed-out terminal at the pump connector, which can mimic a low current condition on one phase.
  • The auxiliary pump may only run during auto-stop events, so a low current fault can be intermittent and not set during a short test drive; a scan tool bidirectional test is often needed to reproduce it.
  • A weak battery or charging system issue can cause the TCM to log this code if voltage drops during pump operation, so always verify battery health before replacing parts.
  • Some vehicles use the auxiliary pump for both stop-start and transmission cooling, and a low current on one phase may not disable the pump entirely but can cause reduced performance without an obvious drivability symptom.
  • The W phase designation refers to one of the three motor phases, and a low current on only that phase often points to a localized winding fault or connector issue rather than a completely failed pump.

Can I keep driving with P0B07?

You can usually drive the vehicle, but avoid relying on the auto-stop feature and be prepared for harsh shifting or a possible limp mode if the pump fails completely. If the transmission begins slipping or refusing to engage, stop driving and seek service.

Tools that help

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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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