I completed a diagnostic on an A/C. The customer stated that after replacing batteries in the thermostat, the system would not do anything. I inspected the system and found no light on inside the furnace. I checked the fuse and it was good. As soon as I put the door back on the system started. I found that the door was loose to the switch causing it not to make a connection. I adjusted the frame of the furnace to compensate for the gap the door has. Cycled and monitored system. All operating normally at this time.