P0B39: Motor Electronics Coolant Pump B Control Circuit Low

P0B39 sets when the control signal of motor electronics coolant pump B is lower than the controller expects, usually a short to ground on the command line or a pump input that pulls the line down. On a low-side switched or PWM line the module sees the voltage stuck low while it commands the pump off. No manufacturer documents P0B39 publicly. Pump B is in service at FCA (Compass and Renegade 4xe, performance code P0C74), and Ford stores the pump A codes P0A06 and P0A07 in the PCM of the 2020 Explorer and Aviator hybrids. CARB requires these inverter cooling outputs to be checked for circuit faults. Open and high faults are P0B38 and P0B3A; running faults use P0C74 and P2D05 ff.

Medium severity. Can I keep driving?Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

Symptoms

MIL and hybrid or coolant warning
Pump runs permanently or not at all, depending on circuit design
Power limitation at high load or heat

Most likely causes

Control wire shorted to ground, often chafed at brackets
Pump electronics pulling the control input low
Coolant or water in the pump connector
Driver stage in the controlling module faulty

What to check, in order

Start with the simplest. You can tick off each step.

With the engine cold, check the coolant level; never do this when the system is hot
With the connector unplugged, check whether the signal wire is making contact with ground.
Check the actual temperature in live data
Make sure the fan comes on when it should
Check the thermostat and connector
Measure the sensor and replace it if its readings do not make sense

Good to know

Repair difficulty: medium
It can trigger limp mode.
It affects emissions.

What each character of P0B39 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
B: Subsystem depends on the code range
39: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0B39?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P0B39?

The most common causes are: control wire shorted to ground, often chafed at brackets; pump electronics pulling the control input low; coolant or water in the pump connector.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.