What Does P0B39 Mean?
The powertrain control module has detected a low voltage condition on the motor electronics coolant pump "B" control circuit. The control signal or feedback voltage is below the expected operating range, indicating a possible short to ground, excessive current draw from a seizing pump, or a wiring fault pulling the signal low.
Safety Warning
Do not drive. A malfunctioning coolant pump risks overheating the power electronics. The vehicle may derate or shut down to protect components.
Common Causes
35%
Short to ground in the coolant pump B control or feedback wiring
30%
Coolant pump B motor drawing excessive current due to seized or binding impeller
20%
Corroded ground connection for the pump circuit causing voltage drop
15%
PCM pump driver transistor partially shorted, pulling the control line low
Diagnostic Steps
1
Step 1: Measure voltage at the coolant pump B connector with KOEO. Check both power supply and control signal voltages against manufacturer specifications.
2
Step 2: Disconnect the pump and measure the control circuit voltage at the PCM connector. If voltage returns to normal with the pump disconnected, the pump has an internal short.
3
Step 3: Measure pump motor current draw while commanding it on via scan tool. Normal draw is typically 2-8 amps; significantly higher indicates a seizing motor.
4
Step 4: Inspect the wiring harness for chafed insulation that could cause a short to chassis ground, especially where the harness passes through body panels or near heat sources.
5
Step 5: Check the pump circuit ground connection for corrosion. Clean and re-torque the ground terminal.
Estimated Repair Cost
$150 - $1,000
Parts + labor, varies by vehicle and location
The powertrain control module has detected a low voltage condition on the motor electronics coolant pump "B" control circuit. The control signal or feedback voltage is below the expected operating range, indicating a possible short to ground, excessive current draw from a seizing pump, or a wiring f...
The most common cause of P0B39 (Motor Electronics Coolant Pump "B" Control Circuit Low) is: Short to ground in the coolant pump B control or feedback wiring
Typical repair costs for P0B39 range from $150 to $1,000, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Do not drive. A malfunctioning coolant pump risks overheating the power electronics. The vehicle may derate or shut down to protect components.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0B39 to identify the root cause.
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Quick Info
Category
Powertrain
System
Hybrid/EV Thermal Management System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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