OBD-II code · Powertrain (generic)
P0E78 Coolant Bypass Valve B Control Circuit Low
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.
Is it safe to drive with P0E78?
The car may go into limp mode (reduced power). Get it diagnosed before a long trip.
Common causes
- Most likelyWire shorted to ground
- PossibleCoil internal short
- Less likelyConnector pins corroded - high contact resistance
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1–5 h
- Emissions test
- Usually unaffected
- Parts involved
- Coolant Bypass Valve B, Wiring Harness
Related codes
- P0E74Coolant Bypass Valve A Control Circuit Low
- P0E75Coolant Bypass Valve A Control Circuit High
- P0E76Coolant Bypass Valve A Performance
- P0E77Coolant Bypass Valve B Control Circuit/Open
- P0E79Coolant Bypass Valve B Control Circuit High
- P0E7ACoolant Bypass Valve B Performance
- P0E7BAuxiliary Cooling Coolant Temperature Sensor A Circuit
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.