01 P0106: Manifold Absolute Pressure (MAP) System Performance • 02 Clutch First or Brake First? The Truth About Stopping Your … • 03 The Ultimate Guide to the Major Automotive Groups and Their… • 04 This Is Why Your Car’s Door Locks Keep Flickering • 05 Tom Cruise Is Back Behind The Wheel: Days Of Thunder 2 Anno… • 06 The Real Reason SUVs Are Dominating the Market • 07 Why Turning Off the Engine Immediately is Bad for Your Turb… • 08 This Rolls-Royce Has An Umbrella That Dries Itself Inside T… • 09 P0031 H02S heater control circuit low (Bank 1 sensor 1) • 10 How to Increase Horsepower in a Naturally Aspirated Engine •

DTC codes / P0Bxx · Hybrid propulsion / P0B01

P0B01Electric/Auxiliary Transmission Fluid Pump Motor Phase U Current Low

P0B01 indicates that the electric or auxiliary transmission fluid pump motor controller has detected an abnormally low current draw on phase U of the three-phase pump motor circuit. The computer compares commanded pump operation against measured current and flags this code when phase U current falls below the expected threshold, pointing to an open circuit, high resistance, or a failing pump motor.

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

Symptoms you may notice

  • Illumination of the malfunction indicator lamp (MIL) on the dashboard
  • Often no symptoms besides the warning light, especially if the auxiliary pump is only used during engine stop events
  • Transmission may enter limp mode or default to a fixed gear if the pump is needed for cooling or lubrication
  • Rough or delayed engagement when the vehicle restarts after an auto-stop event
  • Overheating warning or reduced power if the auxiliary pump fails to circulate fluid during high-load conditions

Common causes, most likely first

  1. Open or high-resistance wiring in the phase U circuit between the pump motor and the control module
  2. Failed electric auxiliary transmission fluid pump motor (winding fault or internal open)
  3. Corroded, loose, or damaged connector pins at the pump or controller
  4. Faulty transmission control module (TCM) or pump control module driver for phase U
  5. Blown fuse or relay supplying power to the auxiliary pump circuit
  6. Internal short or damage in the pump motor harness from heat or chafing

How to diagnose and fix it

  1. Verify battery voltage and inspect the auxiliary pump fuse and relay, replacing any blown fuse or faulty relay
  2. Perform a visual inspection of the pump motor harness and connectors for chafing, corrosion, or loose pins
  3. Use a digital multimeter to measure resistance of the phase U winding and compare to specifications
  4. Check for continuity and voltage drop on the phase U wire from the control module to the pump motor
  5. Command the auxiliary pump on with a scan tool and monitor phase U current and motor speed parameters
  6. If wiring and connectors are good, substitute a known-good pump motor or controller to isolate the fault
  7. Clear codes and perform a road test with auto-stop events to confirm the repair and ensure the code does not return

Phase U current faults that fool technicians

  • A low current reading on phase U often results from a poor ground connection at the pump motor rather than a failed motor, so always load-test the ground circuit before condemning the pump.
  • Some vehicles only run the auxiliary pump during engine auto-stop, so the code may set without any drivability symptom and can be missed during a short test drive.
  • Aftermarket replacement pumps with incorrect winding resistance can trigger P0B01 immediately after installation, so compare new parts to the original specification.
  • The TCM may log P0B01 as a history code after a low-voltage event, so check for other low-voltage codes and clear them before chasing a phantom pump failure.
  • A chafed harness near the transmission mount is a common hidden cause that only shows up when the engine moves under load, so wiggle-test the harness while monitoring current.

Can I keep driving with P0B01?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy loads, trailer towing, or prolonged idling because the auxiliary pump may be needed for transmission cooling. If the transmission enters limp mode or you notice slipping or overheating, stop driving and have the vehicle towed.

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.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage