Two easy ways to tell are if the motor takes too long to warm up. This means the thermostat may be stuck open. If the motor gets hot and the top hose on the radiator is not warm the thermostat may be stuck closed.
less heat in your car
The air conditioner or heater continually runs.
One of the most common signs of a bad thermostat in a Subaru is the vehicle overheats. This is a sign that something is not working right with the thermostat. You may need a new one, or you may be able to repair the issue.
Overheating or no heat from the heater are sure signs it is bad. You car is 12 years old, just replace it if you have a suspicion it is bad. You can test the thermostat but why bother? You will have to remove it to perform the test. Just replace it.
Check for water in your oil, if so, this is a sign of bad head gasket. White smoke from the exhaust and loss of coolant with no apparent leak are all signs of a blown head gasket. You can also remove the thermostat (it is much cheaper to replace and easier to troubleshoot as well). If the temperature of the vehicle lowers, your thermostat was the problem.
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Yes a bad thermostat would cause overheating in any car. A thermostat regulates the temperature.
Thermostat and thermostat sensor (ect) bad
Could be the thermostat not opening
Only if the thermostat doesn't work. This question needs more information.