Look on page six of the troubleshooting manual.
In my opinion, your problem might be the "Flame Sense" circuit on the main board is bad.
It works by looking for ions in the air flow from the flame being lit. No flame, no or "not enough" ions in the air. With normal air flow to the burner the sensor may not be seeing "enough" ions to convince the main board that the fire is lit and shuts down the furnace.
Blocking some of the incoming air might allow more ions to concentrate at the sensor and the main board then is happy and allows the flame to keep going.
This company carries circuit boards for most of our products that use them. Dinosaur Electronics Home. High quality circuit boards for RV appliances.
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