What it means
The coolant bypass valve B reports a control circuit/open condition.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
Coolant bypass valve A directs coolant through or around part of the thermal-management circuit. The control module monitors the valve’s electrical control circuit and sets P0E77 when it detects a high circuit signal while checking or commanding valve A; the listed open wire, open coil, or backed-out pin can interrupt the circuit.
Common causes
Most likely first.
- Open in control wire
- Coil open
- Pin pushed back out of connector body
Symptoms
- Check-engine or hybrid system warning light
- Cabin heating or cooling may be reduced
- Temperature regulation may be slower
How to diagnose it
- Use a scan tool to record freeze-frame data and check valve A command, circuit status, and coolant-temperature data to see when the fault occurred.
- Inspect valve A’s connector and harness for a backed-out pin, loose fit, corrosion, or damaged wiring that could leave the circuit open.
- Check the relevant fuse and power supply, if fitted, against the vehicle wiring diagram before testing the valve circuit.
- With the circuit isolated as specified by the manufacturer, check continuity of valve A’s control wire and inspect for an open between the valve and module.
- Measure valve A’s coil and compare it with the manufacturer’s specification; if wiring and coil pass, follow the manufacturer’s procedure to check the module driver.
Common fixes
- Reseat the pushed-back connector pin and secure the connector.
- Repair an open or damaged valve A control wire.
- Clean or repair corroded connector terminals and restore the seal.
- Replace valve A if its coil is open.
- Repair the control module circuit only if testing confirms a driver fault.
Can I keep driving?
Because this fault can affect coolant routing and may trigger reduced-power operation, avoid demanding driving and watch for temperature warnings. Stop safely if a battery or motor temperature warning appears, or if power drops significantly.
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.