Im having a problem with my heater. When the heater starts you can see the pilot turn on and then ignite the gas, but my fan won't come on, the gas shuts off because it gets too hot for safety reasons, and then, when it cools down the process starts again, pilot turns on, gas ignites, no fan, gas shuts off. The fan is on AUTO when this happens, but if I put the fan on ON, the unit works fine. Please help me. Also one more thing, when the unit shuts off because it gets too hot because the fan won't come on, my programmable thermostat screen goes blank, but once the unit cools down, my thermostat screen comes back on. What's wrong with my unit.