Robertshaw 9420 -no display , unit will not come on inside or outside- replaced fuses outside disconnect , checked all breakers in panel , press the reset button on thermostat, display shows for a second then disappears again - air handler fan will run in the on position with the main thermosat switch is in the off position - all wiring appears intact - no one messed with anything, it just stopped working ?? Bad thermostat or have I missed something ! Help ! R Pittman