Atwood water heater problem

If you open up the cover on the outside, you will see a piece of grey foam with 4 wires going into it. Try shorting them out one pair at a time. One set has red wires, the other brown. These are your high & low temp detectors. If it starts when you short one pair out then that is your problem. If so, the good news is replacements are only about $20 and easy to change, you just peel back that foam. Also what can happen is the retaining rings holding them against the tank can come loose on rough toads. That would be obvious if you peel back the foam.

The circuit board is sealed in epoxy. It that is toast, I would replace it with the dinosaur board equivalent.