Powertrain, generic (extended)

P2678 Coolant Degassing Valve Control Circuit/Open

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

In a sealed cooling system, air and vapour bubbles are carried through a thin vent line into the expansion tank, where they separate from the liquid; a bubble-free circuit avoids local hot spots, as cooling suppliers such as MAHLE point out. Some engines put an electrically switched valve in this degassing path so the controller can close it, for example to speed up warm-up. P2678 reports an open or unspecified fault in the drive circuit of that valve. No manufacturer documents P2678 or its valve publicly, so neither the purpose nor any threshold can be named. CARB requires electrical checks of controlled cooling system components. If the valve fails in the wrong state, warm-up can lengthen or venting may be impaired.

Likely causes

Unplugged connector or broken wire to the degassing valve
Open solenoid coil in the degassing valve
Faulty driver in the control module

Symptoms

  • Check engine light or stored code
  • Longer warm-up or slow heater output
  • Gurgling or air in the cooling system

Repair estimate

DifficultyMediumDIY possible
Parts and labour€40–400
Time0.5–2 h

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

Parts involved

coolant degassing valvesolenoidwiring harness

Related codes

P2679P2680P2681

References: law.cornell.edu, mahle-aftermarket.com.