I diagnosed Shorted contact or for the outdoor unit caused the low voltage fuse to blow. This was causing the thermostat to be blank. I corrected the program in the thermostat to operate for a furnace system set up with straight ac. The system is an air handler with electric heat and straight ac. The thermostat was set up and wired up for a heat pump with reversing valve. I installed the contacts and tested the system. The system is currently functioning in all modes with out blowing the fuse. I also performed a single system heating maintenance today. Customer signed arch form and I gave customer the discount for today’s repairs. I replaced 1-20x25x1 filter. I checked the amp draw on the blower motor and heat strips. I checked safeties and confirmed proper sequence of operation. The system is currently heating at manufacturers specifications.