What it means
The motor electronics coolant pump B control circuit reports a control circuit low condition. Power electronics cooling cannot be commanded reliably on branch B.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Pump B circulates coolant through the hybrid or EV power electronics. The control module monitors the pump's control circuit and sets P0B39 when the circuit signal is lower than expected, which can happen if the wire is grounded or the pump or connector pulls the signal down.
Common causes
Most likely first.
- Control wire shorted to ground
- Internal short in pump motor
- Connector pins corroded - high contact resistance
Symptoms
- Hybrid-system warning light may illuminate
- Reduced power or limited hybrid operation
- Power electronics may overheat or trigger protective shutdown
How to diagnose it
- Record freeze-frame data and check scan-tool pump B command and status to see when the low circuit condition occurs.
- Inspect pump B wiring for chafing or pinched insulation that could let the control wire touch ground.
- Check the connector for corrosion, damaged pins, or a poor fit that could create high resistance and distort the circuit signal.
- Following the manufacturer's high-voltage safety procedure, test the control wire for an unintended short to ground and compare circuit readings with the manufacturer's specification.
- Test pump B for an internal short using the manufacturer's procedure, then check the controller output if the wiring and pump test normally.
Common fixes
- Repair chafed or pinched pump B control wiring that is shorted to ground.
- Clean or repair corroded connector pins and restore a secure terminal fit.
- Replace pump B if testing confirms an internal short.
- Repair the pump control module connection or output if circuit tests isolate the fault there.
Can I keep driving?
Limit driving because pump B may not cool the power electronics, which can cause reduced power or a protective shutdown. Avoid heavy loads and stop safely if an overheating warning or significant power reduction appears.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.