Upon my arrival I performed a 6 month checkup on 1 furnace system. Flushed and treated the water removal system, cleaned the flame safety sensor, verified proper fuel delivery, checked for CO then verified system safety and performance. I found the system performing to manufacture specifications at this time. During the eval I noticed the bacterial prevention system not working. I spoke with the customer and he will purchase a new bulb himself to see if it fixes it and if not he will see about purchasing a new bacterial prevention system himself and installing it unless wiring has to be redone then he will call for out assistance.