Hi All, I have a 14 year old York system that will only start occasionally. Actually, I'm starting to believe it is haunted, and only starts when it feels like it. The unit is located in the attic, and each time it attempts to start it sounds normal, but then shuts back down before it can start blowing heat. I first noticed this a couple of weeks ago, but when I flipped the power to the furnace off and back on again it fired back up like normal. This problem has been happening every couple of days, but pulling the batteries from the thermostat would quickly rectify the problem. The problem suddenly became much worse two days ago, and now I can only get it to start once or twice per day. I have climbed into the attic and watched as the ignitor fires and the gas is ignited, it then sits there for a few seconds and then shuts back down. It kind of seems like the blower just doesn't start. However, the fan will start when switched on at the thermostat. Here's the quick list of things I have tested and/or been able to confirm: [LIST] [*]I replaced the thermostat with a wireless version, which now lets me test from the attic. Although I experience the same problem. [*]The fan will start when the thermostat is set to [B]On[/B]. [*]The AC/cooling will start normally when switched on, even though the inside house temperature is around 60 degrees. [*]I think the heat turned on only once last night, which was obvious because the wife quickly shed the extra blankets. However, I had to quickly retrieve them when the unit would not start again. [/LIST] I'm at a loss here, and hope this is something easily recognizable and reparable. Any help getting me pointed in the right direction will be much appreciated!

:wall: This is a mystery to me. The pilot is lit, the fan works, but no gas gets delivered to the burners, so it fails to heat. I started with the thermostat. I find the fan to work if on. I did the thermostat test, crossing wires W+R, and nothing starts. The thermocouple is keeping the pilot lit. I get low voltage between W and R at the thermostat. And the same on the gas switch before it goes into the metal block. I have even banged on the gas switch to see if I could knock something loose. Nothing seems to work, or even give me a clue as to what is wrong.:mad: I have perused the forum for hours and found more tests to do, but not the solution to the problem. I still do not understand the functional details of the gas switch, and I think I could solve this problem if I could get a description of what each piece does. Or maybe a manual, blowup chart, or any other clue. This monster is so old I could not even tell you what brand it is. But I can desribe it for you. Simple parts. The air flow is horizontal thru the unit, over a plate that is warmed by the burners underneath. The burners have a pilot light, maintained on by a thermocouple. The fan motor is before the hot box, and blows when the hot box reaches the correct temperature. I have replaced the fan motor once. It works much better than the original. The thermocouple is old, but if it keeps the pilot lit, it should not need replacing, right?! And the thermocouple should not prevent the gas from flowing from the main, right?! Please do NOT ask me if the gas line is on. I have half a brain left. So what is in the gas switch that would suddenly prevent the gas from flowing to the burners? This is not a modern efficient unit that has an actuator fan, or anything else fancy. Before I start replacing parts to try and fix it, I really need to know what is in the switch and how it works. I can give you a picture if necessary. To quote Leeloo, "Help ... please ... help." :help: