The thermostat can Fail OPEN in which cause you will have NO or very little heat in the car. The thermostat can Fail CLOSED in which case the engine can overheat OR run hotter than it should... Care must be exercised with the cooling system I remove the radiator cap when its COOL start the vehicle let it reach operational temperature and view if coolant is flowing properly... '07 never a mistake to simply replace it especially on a P071 these vehicles have had extreme use. Regards
Pull the thermostat if before u start it up if it the brass center is open then it is stuck open before replacing the thermostat westher it is stuck open or closed do a full system flush most times when thermostats fail it is because of there being gunked up oldass coolant u need to fulsh the system properly then change the stat
Thermostats can be tested by placing them in water and heating the water while monitoring the temperature and watching the thermostat to see at what temperature it opens. This test is NOT worth the trouble as thermostats are pretty low cost and fairly easy to replace on most vehicles.
The last time i replaced a stat in my 302 i think it was under $20 to complete the repair
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