P0E79: Coolant Bypass Valve A Control Stuck/Open

P0E79 reports that bypass valve A apparently remains open although the module has asked it to close. The control unit cannot see the valve directly; it infers the position from temperatures, typically by comparing two coolant sensors that should drift apart when the loops are separated. No manufacturer documents P0E79 publicly. Ford's 2019 hybrid summary describes exactly this logic for its older code P26AB, a valve stuck in the combined loop: if the heater-loop sensor reads more than 5 °F above the engine sensor within 60 seconds after a circuit fault, the valve is taken as stuck. An open valve lets heat escape into the larger loop, so cabin heat in electric mode suffers and the engine may warm up differently. CARB requires functional monitoring of such outputs. The opposite case is 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

Weak cabin heat in electric mode
Engine starts more often to supply heat
Check engine light

Most likely causes

Valve A jammed open by debris or a broken internal part
Actuator no longer moves the valve
Hoses swapped at the valve after a repair
Skewed coolant temperature sensor misleading the check

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
Check the continuity of the wire between the component and the control module.
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 P0E79 means

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

Frequently asked questions

Can I keep driving with code P0E79?

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 P0E79?

The most common causes are: valve A jammed open by debris or a broken internal part; actuator no longer moves the valve; hoses swapped at the valve after a repair.

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.