P2678: Coolant Degassing Valve Control Circuit/Open
What to do
Nothing dramatic will happen straight away, but this is worth attending to rather than leaving.
This valve lets air escape from the cooling system. Modern engines route coolant through many branches with high points where air collects and cannot get out by itself, and trapped air causes local hot spots the temperature gauge cannot see — because the gauge's sensor is sitting in liquid somewhere else.
Two things worth watching and reporting: a temperature gauge that moves up and down faster than seems reasonable, and a coolant level that keeps needing topping up with no puddle anywhere. The second one especially — if the level keeps dropping and a pressure test finds no leak, that is usually air working its way out rather than coolant escaping, and it points at this rather than at a hunt for a leak that is not there.
The fault is usually the connector or the wiring rather than the valve itself.
What this code means
An open circuit in the control of the valve that vents trapped air out of the cooling system.
Air in a cooling system is not a harmless bubble, and understanding why is the point of this page.
A modern engine's coolant does not follow one loop. It runs through the cylinder head and block, a radiator, a heater matrix, usually a turbocharger bearing housing, an exhaust gas recirculation cooler, sometimes an oil cooler and a gearbox heat exchanger — a branching network with high points in several places. Air collects at those high points and cannot get out on its own, because coolant flows past a pocket rather than through it. A degassing valve is an actively opened vent that lets those pockets escape to the expansion tank.
What trapped air actually does, in order of how much it costs:
- It insulates. Air carries heat away roughly three orders of magnitude worse than coolant, so a pocket resting against a cylinder head is a local hot spot in an engine whose gauge reads perfectly normal — because the sensor is sitting in liquid somewhere else.
- It lies to the temperature sensor. A sensor with air around it reports the air, not the coolant, and the module then makes fuelling, fan and thermostat decisions on a number that is wrong. A temperature reading that swings implausibly fast is a classic sign.
- It compresses. A pump generating pressure against a compressible pocket moves less coolant than its speed suggests, so flow falls exactly where it is already marginal.
The practical presentation is worth knowing because it sends people looking for the wrong thing: a cooling system that needs topping up repeatedly with no leak anywhere. Each time the engine works air out of a pocket, the level in the expansion tank drops, and the natural conclusion is a leak nobody can find. A pressure test that holds while the level keeps falling is that story, and it points at air rather than at a leak.
With the valve's circuit open it cannot be commanded, so air accumulates rather than being vented. Nothing fails immediately, which is why this is moderate rather than severe — but it is a fault that gets worse rather than staying put.
Likely causes
Start here
- Corroded or loose connector · easy
- Open circuit in module wiring · moderate
- Faulty coolant degassing valve · moderate
Less common
- Air trapped in cooling system · moderate
- Damaged wiring harness · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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