What it means
The coolant bypass valve B reports a control circuit low 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 routes coolant through or around a thermal-management path. The module monitors its control circuit and sets P0E78 when the circuit response is outside the expected range; a short to ground, internally shorted coil, or high-resistance corroded connection can cause that mismatch.
Common causes
Most likely first.
- Wire shorted to ground
- Coil internal short
- 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 inspect live valve A command, circuit status, and coolant temperatures to identify the conditions when the range fault appears.
- Inspect valve A’s wiring and connector for chafing, moisture, corrosion, loose terminals, or pin push-back, especially where the harness moves or contacts nearby parts.
- If the fault is active, gently move the accessible harness while watching circuit status to check for a changing connection; avoid disturbing high-voltage components.
- Using the wiring diagram and manufacturer’s isolation procedure, test the control wire for a short to ground and check connector contact condition.
- Measure valve A’s coil and compare it with the manufacturer’s specification; if the coil and wiring pass, check the module output according to the manufacturer’s procedure.
Common fixes
- Repair chafed valve A wiring and restore its insulation and routing.
- Dry, clean, or replace corroded connector terminals and renew damaged seals.
- Repair a control wire short to ground.
- Replace valve A if its coil is internally shorted.
- Repair the module output only if circuit testing identifies it as the cause.
Can I keep driving?
Limit driving if the vehicle shows reduced power or a battery or motor temperature warning, since incorrect coolant routing can affect thermal control. Stop safely if a temperature warning appears or the vehicle’s power falls sharply.
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.