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Allot of things to look at here below is the Atwood troubleshooting for the type of situation you are having. It sounds like the flame sensor is not sensing the flame. When this happens the gas valve closes so the flame goes out. Then the circuit board tries again to relite, and eventually goes into lockout. (3 strikes and you're out.)The ignition probes and flame sensor need to be removed and cleaned and then see what happens. Check the igniter for any cracks.
BURNER IGNITES BUT IGNITION SYSTEM "LOCKS OUT" AND TURNS BURNER OFF
I am going to see if I have a manual laying around to better assist. This may take a few. Pretty much once you see the igniter you should see the flame sensor and flue, the flue can be blown out with low pressure air, the flame sensor and ignitor can be cleaned with steel wool, let me see if I have more info.
I am sending you a PDF which is allot of information because it covers all models. There is a breakdown of the 8940 near the end. According to this information the control board detects the presence of flame. IF this is not what you are looking for I can opt out for another expert. There is allot of great information and explains how each component works. You can view the PDF by clicking HERE.
Yes this is correct it is accessed by pulling the outside panel down and removing components. I did not mean to overwhelm you with so much information, I was hoping the breakdown of the furnace would give you an idea what you where up against. IF you do not feel comfortable pulling it apart I would have a RV shop tackle this project, cleaning everything may not even solve the issue. Normally I would expect an hour worth of labor for cleaning and troubleshooting the issue.