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DTC codes / P0Bxx · Hybrid propulsion / P0B38

P0B38Motor Electronics Coolant Pump B Control Circuit/Open

P0B38 indicates that the engine control module has detected an open circuit or abnormal voltage on the control circuit for the electric coolant pump that cools the motor electronics, often part of a hybrid or electric vehicle's thermal management system. The computer monitors the command wire to the pump and sets this code when the expected electrical signal is missing or out of range, which can prevent the pump from running and lead to overheating of power electronics.

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

Symptoms you may notice

  • Illumination of the check engine light or service vehicle soon message, often with no other noticeable symptoms
  • Reduced power output or limp mode triggered by the vehicle's protection strategy for overheated electronics
  • Overheating warning message or temperature gauge climbing above normal, especially during sustained driving or charging
  • Cooling fan running excessively or continuously as the system tries to compensate for a non-functioning pump
  • Inability to start or charge the vehicle in some hybrid and electric models due to thermal management faults
  • Audible warning chimes or a message to pull over safely when the electronics temperature exceeds a threshold

Common causes, most likely first

  1. Failed electric coolant pump motor or internal control module, which is the most common cause of an open circuit on the control side
  2. Damaged or corroded wiring harness between the engine control module and the pump, often from heat exposure or chafing
  3. Loose, corroded, or disconnected electrical connector at the pump or at the control module
  4. Blown fuse or faulty relay that supplies power to the pump control circuit
  5. Engine control module software issue or internal driver failure, though this is less common
  6. Aftermarket or incorrect pump installed that does not match the vehicle's control protocol

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related codes in other modules, then clear the code and test-drive to see if it returns
  2. Inspect the coolant pump connector and wiring for corrosion, loose pins, or physical damage, and repair as needed
  3. Check the fuse and relay that supply power to the coolant pump circuit, replacing any failed components
  4. Measure resistance and voltage on the pump control circuit according to the manufacturer's service information to confirm an open circuit
  5. Command the coolant pump on with a scan tool and listen for operation; if it does not run, test the pump directly with a known-good power source
  6. If the pump fails to operate or the circuit tests open, replace the electric coolant pump and bleed the cooling system properly
  7. After repair, clear codes, verify the pump runs when commanded, and confirm the code does not return during a road test

When the pump tests good but the code returns

  • A pump that spins freely when bench-tested can still set P0B38 if its internal control electronics fail intermittently under heat, so always test the pump while it is hot and installed.
  • The control circuit often shares a ground with other sensors, and a poor ground can mimic a pump failure; check ground points and voltage drop on the ground side before condemning the pump.
  • Some vehicles require a coolant pump run-in or adaptation procedure after replacement, and skipping it can leave the code stored even with a new pump.
  • The motor electronics coolant pump is separate from the engine coolant pump on hybrid vehicles, so a code for pump B does not mean the main engine pump is faulty.
  • A low coolant level or air pocket in the system can cause the pump to overheat and set a control circuit code, so always check coolant level and bleed the system first.

Can I keep driving with P0B38?

It is not safe to continue driving for long distances with this code because the motor electronics may overheat and cause a sudden loss of power or damage. Limit driving to a short distance to a repair shop, and avoid heavy acceleration or high-speed driving until the fault is repaired.

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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