P2678 — Coolant Degassing Valve Control Circuit/Open
OBD2 code P2678 flags a problem with the engine cooling and temperature control (coolant degassing valve control circuit/open). Short, gentle driving to a shop is usually possible. Common culprits: water pump or radiator issues, wiring faults in the sensor circuit, low coolant level or air in the system.
powertrain (engine/transmission)
Is it safe to drive?
Drive with caution Drive with caution and watch the temperature gauge. A thermostat stuck open mainly costs fuel; any sign of overheating means stop immediately — driving an overheating engine destroys head gaskets and worse.
How to diagnose with an OBD scanner
- Read the code and freeze frame Connect an OBD2 scanner, confirm P2678 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
- Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
- Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the engine cooling and temperature control for damage, corrosion, chafing or poor contact.
- Subsystem check Watch coolant temperature in live data during warmup — it should climb smoothly to operating temperature.
- Subsystem check Check coolant level and feel upper/lower radiator hoses to verify thermostat operation.
- Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P2678 does not return.
Approximate repair cost
Repair cost depends on which component actually failed. Typical scenarios for P2678 (parts and labor, USD, independent shop rates): Replace the coolant temperature sensor: $100–$300; Cooling system repair (pump, fan, radiator): $300–$1,200; Replace the thermostat: $150–$400. Overall, fixing P2678 usually costs between $100 and $1,200. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.
Common questions
What does OBD2 code P2678 mean?
OBD2 code P2678 flags a problem with the engine cooling and temperature control (coolant degassing valve control circuit/open). Short, gentle driving to a shop is usually possible. Common culprits: water pump or radiator issues, wiring faults in the sensor circuit, low coolant level or air in the system.
How much does it cost to fix P2678?
Replace the coolant temperature sensor — $100–$300; Cooling system repair (pump, fan, radiator) — $300–$1,200; Replace the thermostat — $150–$400. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.
How do I diagnose P2678 myself?
Start with a scan: confirm P2678, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the engine cooling and temperature control, and verify the suspect component against live data before buying parts.
What is the most common cause of P2678?
The most common causes are: water pump or radiator issues, wiring faults in the sensor circuit, low coolant level or air in the system.
Will P2678 clear itself?
Sometimes P2678 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.
What does P2678 mean?
P2678 (Coolant Degassing Valve Control Circuit/Open) is stored when self-diagnostics detect abnormal readings or behaviour of the engine cooling and temperature control. It belongs to the powertrain (engine/transmission) category.
This code does not appear from a single bad reading: the module must see the fault condition repeat under defined operating conditions before it turns on the warning light. The freeze frame captured with P2678 — engine load, rpm, temperature, speed — tells you exactly when the problem occurs, so reproduce those conditions when testing.
Common symptoms of P2678
- increased fuel consumption
- Check Engine Light on
- temperature gauge reads low or never reaches normal
- poor cabin heat
- overheating warnings in severe cases
What causes P2678?
- thermostat stuck open or closed
- wiring faults in the sensor circuit
- faulty coolant temperature sensor
- failed cooling fan or fan relay
How to diagnose P2678
- Connect an OBD2 scanner, confirm P2678 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
- Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
- Visually inspect the wiring harness and connectors related to the engine cooling and temperature control for damage, corrosion, chafing or poor contact.
- Watch coolant temperature in live data during warmup — it should climb smoothly to operating temperature.
- Check coolant level and feel upper/lower radiator hoses to verify thermostat operation.
- Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P2678 does not return.
How to fix P2678
Start with the cheapest and most common causes before replacing expensive parts:
- Replace the coolant temperature sensor — $100–$300
- Cooling system repair (pump, fan, radiator) — $300–$1,200
- Replace the thermostat — $150–$400
How much does it cost to fix P2678?
Repair cost depends on which component actually failed. Typical scenarios for P2678 (parts and labor, USD, independent shop rates):
- Replace the coolant temperature sensor: $100–$300
- Cooling system repair (pump, fan, radiator): $300–$1,200
- Replace the thermostat: $150–$400
Overall, fixing P2678 usually costs between $100 and $1,200. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.
Can you drive with P2678?
Drive with caution and watch the temperature gauge. A thermostat stuck open mainly costs fuel; any sign of overheating means stop immediately — driving an overheating engine destroys head gaskets and worse.