I have an Apollo Hydroheat gas water heater, part number A650N, which was manufactured in 2004. I have had the same issue for the past three years. For some reason the water heater shuts off. If I manually throw the on/off switch the heater will work for a day or two and then fail again. I have had the unit "repaired" several times. The repair consists of the tech cleaning the burner unit. The last tech said that I had a faulty flame sensor and it needed to be replaced. But when I go on-line and look at the schematics I do not see a flame sensor. Any suggestions.