What it means
The coolant bypass valve B reports a control circuit high condition. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
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 controls coolant flow through or around part of the thermal-management system. The module monitors the valve A control circuit and sets P0E79 when it detects a high signal; a wire shorted to battery voltage, an open coil, or a poor connector connection can produce that condition.
Common causes
Most likely first.
- Wire shorted to B+
- Coil open
- Connector pins corroded - high contact resistance
Symptoms
- Check-engine or hybrid system warning light
- Cabin heating or cooling may be reduced
- Temperature regulation may be slower
How to diagnose it
- Record freeze-frame data and review live valve A command, circuit status, and coolant temperatures to see when the high signal occurs.
- Inspect valve A’s connector and harness for damaged insulation, a wire contacting a power feed, corrosion, or loose terminals.
- Check the relevant fuse and circuit routing against the wiring diagram, then inspect accessible sections for a short to voltage.
- Following the manufacturer’s isolation procedure, test valve A’s control wire for a short to B+ and check connector contact condition.
- Measure valve A’s coil against the manufacturer’s specification; if the circuit and coil pass, check the module driver using the manufacturer’s procedure.
Common fixes
- Repair wiring that is shorted to battery voltage and secure it away from power feeds.
- Clean or replace corroded connector terminals and restore the connector seal.
- Repair loose or damaged valve A wiring connections.
- Replace valve A if its coil is open.
- Repair the module driver only if testing confirms it is faulty.
Can I keep driving?
Avoid strenuous driving if reduced power or a battery or motor temperature warning appears, because valve A may not regulate coolant as intended. Stop safely if a temperature warning comes on or the vehicle loses substantial power.
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.