P0E78 code meaning
Coolant Bypass Valve A Control Circuit Range/Performance
Powertrain system Generic code P0Exx codes Check engine light
What P0E78 means
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.
P0E78 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P0E78
- Check engine light
- Heating output varies from trip to trip
Common causes of P0E78
- Most likelyDeposits or wear making the valve A mechanism sluggish
- PossibleHigh resistance at connector or ground of the valve circuit
- Less likelyLow supply voltage to the actuator under load
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Coolant bypass valve aWiring harness
What each part of P0E78 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 78 | The exact fault in that area. |
What to do next
- Note P0E78 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0E78
What does P0E78 mean?
P0E78 means "Coolant Bypass Valve A Control Circuit Range/Performance". P0E78 is the plausibility code for the actuator circuit of bypass valve A. It is a generic code, so it means the same on every make.
Is it safe to drive with P0E78?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0E78 fail an emissions test?
Yes, it can. P0E78 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0E78 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P0E78 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.