I arrived at the customer's home to find that the low voltage fuse on the furnace control board had blown. After speaking with the customer, I discovered that they had replaced the batteries in their thermostat and accidentally popped the fuse when they reattached it. I checked for any shorts but could not find any. I left the furnace running for 20 minutes and it operated properly. I informed the customer that since they were a member of our service club, we would bill them if they owed us anything.