first step to diagnosis. when u are driving, do u have heat?
if ur temp is at C and ur heater is blowing cold air, most likely thermostat,
on the other hand, if it is blowing hot air, it shouldnt be the thermostat.
even if a fan relay had problem and the fan contantly blows, ur thermostat will close the coolant passage to prevent cold coolant from coming into the engine.
