If the previous posts don't solve the problem, I would check the sail switch in the fan housing. It must prove a certain amount of air flow to allow the ignition to proceed. They can get bent and not sense properly, hang up by rubbing the housing,or simply not make contact when required. After you start the fan, you should hear a click which would be the sail switch closing the ignition circuit and opening the gas valve.
Not familiar with your rig but with mine, I need to pull complete furnace out to open it up for repair.
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