My furnace will cycle on and spark the igniter but no fire. I followed the trouble tree and it said to check for voltage at the valve. The valve has two commons and a blue wire and yellow wire going into the solenoid. The blue wire gets 24v and the yellow does not. The trouble tree said to replace the board, which I did. It acts the exact same. Should the yellow wire at the valve have 24v? It doesn't say specifically.
Check the roll out sensor.
Basically, you can jumper all of the safety sensors with a paperclip one at a time until it fires. Whichever one is holding it out is your issue.
If no fire whatsoever, check the gas train. Does any gas leak out if you Crack the line to the valve body?
In reply to gearheadmb :
Tracing the wiring diagram back it sure looks like the yellow wire is the common or neutral for the control circuit based on where terminated on the control transformer and that the control fuse is in the red wire at the transformer. If you have 24 V to ground (measured between the blue wire at the gas valve and the green ground wire right there) it is possible the gas valve is bad. The fact that you have ignition spark and 24 V at the gas valve would indicate that all the control safeties are closed / normal condition.
After you verify the voltage to ground try pushing the blue wire onto the gas valve terminal and see if you feel or hear any noises from the gas valve, the one on my boiler makes a pretty noticeable clunk when it opens. That appears to be a Honeywell gas valve so there should be numbers on it to source a replacement if it does test bad. If you do try to change it yourself, shut the power off until you've completed replacing the valve, use pipe dope on the pipe threads not teflon tape (bits can break off and jam the gas valve open or so we were taught) take your time and soap bubble any pipe joints to check for leaks. It will take several times to purge air thought the gas valve itself before it will light, if there is a pipe union close by up stream I would recommend cracking that with the gas shut off valve turned on until you smell gas and then re-tighten it / bubble it. Good luck!
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