When I came home my thermostat was in heat and my air conditioning condenser was running. I pulled the disconnect out of the outside electrical disconnect box and the condenser shut off. My furnace burners will light, but the fan does not come on when the thermostat is in auto. I turn the thermostat to fan on and the fan runs, the burners light and start warming the house, but before I get to the desired temperature the burner and fan go off. Any ideas on these problems.