Not functioning properly If stuck closed engine will overheat--no coolant flow through engine. If stuck open--vehicle will not warm up to normal operating temperature - no heat in passenger compartment--poor fuel economy
Must be sticking, replace the thermostat.
I would start with checking your thermostat and thermostat housing. See if its regulating. If it is not replace the thermostat.
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Replace your thermostat.
remove old thermostat and replace with new thermostat and gasket
Replace your thermostat whenever you replace the coolant and service the cooling system. Also replace it if you suspect it is sticking.
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I would replace the thermostat. This sounds like the thermostat is stuck open.
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