P0E77: Coolant Bypass Valve A Control Circuit High

P0E77 is logged when valve A's drive line carries more voltage than the driver should allow, typically because the wire has rubbed through against a 12 V feed. A low-side driver then sees battery voltage while it is trying to pull the line down and usually reports overcurrent; with some designs an open line can float high and end up here too. In either case the valve no longer follows commands reliably. No manufacturer documents P0E77 publicly. Ford's 2019 hybrid summary handles the same situation on its bypass valve as P2683, again set after more than 5 seconds of driver fault status. CARB expects such outputs to be checked for circuit faults. A grounded line is P0E76 and an interrupted one P0E75; replacing the valve without checking the harness first rarely helps.

Symptoms

• Check engine light
• Cabin heat unstable or missing

Most likely causes

• Control wire shorted to a battery or ignition feed
• Open circuit that the driver interprets as high
• Faulty valve A actuator electronics

What to check, in order

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

1. With the engine cold, check the coolant level; never do this when the system is hot
2. Check the sensor's ground and make sure the signal wire is not receiving voltage from another circuit.
3. Check the actual temperature in live data
4. Make sure the fan comes on when it should
5. Check the thermostat and connector
6. 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 P0E77 means

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

Frequently asked questions

Can I keep driving with code P0E77?

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 P0E77?

The most common causes are: control wire shorted to a battery or ignition feed; open circuit that the driver interprets as high; faulty valve A actuator electronics.

Related codes

• P0E75Coolant Bypass Valve A Control Circuit/Open
• P0E76Coolant Bypass Valve A Control Circuit Low
• P0E78Coolant Bypass Valve A Control Circuit Range/Performance
• P0E7DCoolant Bypass Valve B Control Circuit High
• P2683Engine Coolant Bypass Valve A Control Circuit High

Sources

• fordservicecontent.com: OBDSM1901 HEV
• law.cornell.edu: 13 CCR 1968.2

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.