Powertrain, generic

P0E79 Coolant Bypass Valve A Control Stuck/Open

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

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

Symptoms

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

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€150–700
Time1–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

coolant bypass valve acooling system

Related codes

P0E7AP0E78P0E7FP26AB

References: fordservicecontent.com, law.cornell.edu.