P0E78: Coolant Bypass Valve A Control Circuit Range/Performance

P0E78 is the plausibility code for the actuator circuit of bypass valve A. Neither an open nor a short is present, yet current, feedback or response time of the actuator do not fit what the module has commanded: a coil whose resistance has drifted, a motor that labours against a sticky shaft, or a supply that sags under load. For motor-driven valves with position feedback the module may also see the valve arrive late or stop short. No manufacturer documents P0E78 publicly. Ford's hybrid summary for 2019 shows the general idea of checking the actual valve position once an electrical problem is known, under P26AB. CARB asks for functional checks of output components in addition to circuit tests. Hard electrical faults are P0E75 to P0E77; a valve that stays put is P0E79 or P0E7A.

Medium severity. Can I keep driving?Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

Symptoms

Check engine light
Heating output varies from trip to trip

Most likely causes

Deposits or wear making the valve A mechanism sluggish
High resistance at connector or ground of the valve circuit
Low supply voltage to the actuator under load

What to check, in order

Start with the simplest. You can tick off each step.

With the engine cold, check the coolant level; never do this when the system is hot
Compare the reading with other engine data while varying the engine speed.
Check the actual temperature in live data
Make sure the fan comes on when it should
Check the thermostat and connector
Measure the sensor and replace it if its readings do not make sense

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P0E78 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
E: Subsystem depends on the code range
78: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0E78?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P0E78?

The most common causes are: deposits or wear making the valve A mechanism sluggish; high resistance at connector or ground of the valve circuit; low supply voltage to the actuator under load.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.