I am working on a Lennox Superior gas fireplace, similar to their model SDV35 (although the exact model on the fireplace I can’t find online anywhere: DT-400CMN-J) It has the millivolt system (manual spark pilot igniter with a ‘three flame’ pilot, a thermocouple and a thermopile). Looks like this: It has a SIT Millivolt gas valve. I know the purpose of the thermocouple and thermopile (to generate electricity when heated), more by the thermopile, but I’m not totally clear on what affect each has on the gas valve’s operation. Symptoms are the pilot will light and stay lit as long as the gas valve knob is in the Pilot position, even when not held in/down. However as soon as it is turned to the On position the pilot goes out (and of course the burners do not come on). In that the pilot stays on in the Pilot position, I’m thinking that this is not a thermocouple issue, but rather may be a thermopile issue? But does the gas valve need electricity from the thermopile just to keep the pilot on when in the On position? Conversely, is it possible for the thermocouple to be not working such that it will keep the pilot on when in the Pilot position but not when in the On position? The pilot flame seems well wrapped (say 60%) around the thermocouple and thermopile and I cleaned off both. Although I didn’t measure the voltage coming from the thermopile (understand it should be somewhere around 400 millivolts) as I didn’t have the appropriate meter with me when I was there. I will do that when I go back. Any advice on how to troubleshoot this problem is appreciated.