heater fan goes off on high, what needs replaced ?

Usually when the blower does not work on high it is a faulty blower switch. Though it is merely a guess, the better thing to do would be to inspect the power coming out of the switch and the blower resistor to see what the exact reason is. Somewhere there is a loss of voltage causing your problem. High speed sometimes has it's own circuit and protection. Feel free to stop by for assistance if needed. -Curt's Service Team 248-545-0500