Atwood Furnace Problems

So I’m having the same problem as the OP was having... I have replaced the circuit board, sail switch and igniter... I also replaced the gas regulator, nothing to do with the heat issue just didn’t like the one that was on there. It’s an Atwood 8535. I took the screen part out where the gas flowes through, there was some rust blocking a few places, wire brush cleared that up. I put everything back together and still having the same problem... fan comes on, igniter lights and the heat blows briefly and kicks off, repeats two more times and shuts down, light blinking 3 times. I don’t know what else it could be??