Thread: coolant problem and radiator

ive heard people say a head gasket cause of bubbles. but when its running for the first 5-10 minutes theres no bubbling. and theres no check engine light on. this is my friends car and a month ago she had the t-stat replaced by an unknown person not in a shop. if they put one in too cold would that be a cause? so if the coolant isnt "boiling" why would it be bubbling? the temp gauge in the car is reading cold. and doesnt raise. i replaced the radiator cap but no luck. you think a bad radiator is causing all of this? or is it fine but not turning on becuse its reading cold. also, if theres air in the system will it cause this?