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Do you have a volt meter to see if we are getting 24 volts too the gas valve?
OK is the pilot a glow bar ?
I am not having much luck finding a manual on the furnace. Does it have a control board?
Is there a wiring diagram you can send me a pic of? And on call for heat see if we are getting 24v between the Common and Main valve terminal on valve?
Lets check between MV-MV/PV for 24 volts
Make sure all the connections between the module and gas control are clean and secure, Try unplugging and re plugging as they can develop corrosion and not make good contact.
Are you saying the pilot does not light everytime or it just turns on and off during the same call for heat?
Based on this I am suspecting the module is going to need to be replaced.
Erratic behavior as well as not ever getting the 24v out to mv once the pilot valve has opened is characteristic of a failing board.
I can open this to other experts to see if they have more ideas if you would like.
Below is my flowchart for the 8660
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