check furnace ignition
If your furnace doesn't always ignite reliably even though you can hear the ignitor clicking and can smell gas when the valve opens, wait until after dark, open the furnace doors, and watch carefully for sparks flying around as it tries to ignite. Have a flashlight ready in your hand and turn it on quickly to see where the spark is jumping to. Now take the furnace apart again for the gazillionth time and rearrange the insulation on the high tension wire so the spark can't jump across there. When you put the furnace back together and check for gas leaks with a spray bottle of soap suds be careful not to get soap suds on the high tension wire or you'll have to take the whole darn thing apart again one last time to dry out the wire... Doh! ...but it'll probably work if you've got fuel and ignition ...now that's not to say anything about the mud dauber wasps that may have plugged up everything but if you're like me you already took the furnace apart to clean up all that stuff before you got to this puzzling point of no ignition. Hopes this makes your life a little warmer ...or prevents you from ending your own life or that of your frustrating furnace, not that I considered that a time or three 
Topic: check furnace ignition