Thermostat stuck open

Symptoms

• Temperature gauge reads lower than normal or takes a long time to reach normal operating range
• Heater blows only lukewarm air, especially at idle or in cold weather
• Check-engine light may come on for a cooling system-related code on some vehicles
• Poor fuel economy in some cases due to the engine not reaching efficient operating temperature

Likely causes

• CommonThermostat mechanically stuck in the open positionA spring or valve failure can leave the thermostat unable to close, allowing constant coolant flow to the radiator.
• PossibleMissing thermostat (removed and not replaced)Some past owners remove thermostats attempting to prevent overheating, which causes this exact problem instead.
• CommonOld thermostat wearing outLike other wear items, thermostats can fail gradually as they age.

What to do

1. Check how long the temperature gauge takes to reach its normal operating position; unusually long warm-up times point to this issue.
2. Check heater performance, especially at idle in cold weather, since weak heat is a classic sign.
3. Get any check-engine light codes read, since some vehicles set a specific code for a thermostat that isn't regulating temperature correctly.
4. Avoid assuming this is a safety issue, since it usually just affects comfort and efficiency rather than causing damage.
5. Have a mechanic replace the thermostat, checking that one is actually installed if it was previously removed.
6. Use the exact thermostat rating (temperature) specified for your vehicle, since a lower-rated 'performance' thermostat can cause the same symptom.
7. Confirm normal warm-up time and heater performance after replacement.

Tools and parts

• Coolant (for refill after repair)

When to call a pro

See a mechanic for thermostat replacement, since it involves draining and refilling coolant properly and confirming the correct thermostat rating for your engine.

Safety