No heat on my mitsu montero sport 2001

Either you have an air lock in the heater core or is leaking. first off pull the carpet around the floor and see if its wet or damp, also look for water marks to indicate water leakage, if the floor is wet then you have a leak.If its an air lock you need to raise the front of the vehicle so that the radiator cap is the highest point in the car, jacking it up usually works.start the car and turn the heater on to hot, warm the car up to normal operating temp then just fill the radiator and the air bubbles should come out.If you do this and you see a heap of small bubbles continually coming up really fast, then you have either a cracked head or a blown head gasket