When I arrived, ac was running and cooling. I inspected customer installed contactor. Contactor was installed correctly. Checked wiring and operation, system is operating normal at this time. Customer informed me that he hooked low voltage wires up on same side of contactor and blew the low voltage fuse on furnace. When he replaced fuse, his Nest thermostat showed an error of ac equipment not connecting to thermostat. This error does not allow him to access his thermostat options. Checked for correct wiring on thermostat terminal strip. System is wired correctly. On line app shows that thermostat is off line. I quoted T6 WIFI(350.00+tax) and Lyric thermostat (448.00+tax), he is not interested in having us replace thermostat. He will get new thermostat and wire it himself. System is operating normal at, with exception of thermostat, at this time. Job complete.