I visited a home where the customer had a strange issue for July: their AC wasn't working, but the indoor furnace fan wouldn't shut off. I traced the problem to the furnace's control board, which had failed and was no longer correctly communicating with the thermostat or the outdoor unit. I solved the problem by replacing the faulty control board with a new one. This immediately restored normal function, allowing the air conditioner to run properly just in time for the weekend.