Brian, It's a common problem that the 4th speed in the heater fan/blower motor will shut off. You can replace the circuit breaker and it will do it again when on the fourth speed. I wouldn't advise uping the current on the circuit breaker, just don't run the fans on 4th speed. Use 3rd as max. It won't hurt for a while then when they shut off goto 3rd. 3rd and 4th go thru the CB and 1st and 2nd go from the switch. You also have a relay for 3rd and 4th ea. John hervey http://www.specialtauto.com/ << Does anybody have an idea why my heater fan shuts off when running the ac.I'll have it on high then it with stop blowing or will blow on low,if you turn the fan switch to low then back to high it will work usauly.It does this quite a bit but not all the time.Could this be a bad switch or some type of power falure?Any thoughts would be appreciated v >>