How do you know when your thermostat and head gasket need to be replaced?

The thermostat needs replacing any time you service the cooling system. You should also replace it if the engine is overheating, and you suspect it is defective. A blown head gasket will exhibit, certain symptoms. Coolant in the oil, white smoke from the tailpipe, overheating, air bubbles coming from the radiator, missing on one or more cylinders, a brown looking foam on the underside of the oil cap. A compression test will help to verify this.