Furnace won’t start

[KamFlyer]

One other item that is frequently overlooked when troubleshooting this particular symptom is the over temp limit switch located on the back of the furnace. In the SF35Q it is series connected to the sail switch. The limit switch is normally closed and shouldn't cycle except during an over temperature condition. This is usually caused by blocked heater vents or a blocked cold air return resulting in reduced air flow.
It is a bit of a challenge to access as the furnace usually needs to be removed to do so. It can be tested with a multimeter though.
I have attached a copy of the schematic for anyone interested.
-Rob-

[howson]

KamFlyer -- the OP stated the fan comes on ( post 13 ) and that there is spark (attempt at ignition). The board won't send ignition spark on this furnace (it's model specific!) if the sail switch or limit switch are open. (Control board must sense 12v on the red wire, top of control board, before sending a spark to the electrode.)

Based on the stated symptoms, it shouldn't be the limit switch. If it was the heater would act like it had a sail switch that wasn't closing.

[Jlawles2]

One other thing to consider is wind. Should the ports for the heater be pointed into the wind, it could cause an over air situation where there is not enough fuel to oxygen ratio to support combustion. Does not happen often, but something to consider.