There is a flame rod that may need to be cleaned. It is a metal probe that snese the flame. Has ceramic on the end and a metal probe. Lightly sand that ans see what happens. looks somthing like this
I tried that, but the heater still went out after a few minutes.
There only only two things that would dhut it doen one is the flame rod, or the main control board. Without a meter its a crap shoot but the flame rod is cheap and easy to replace. here is the Grainger parts list
Both, you should have at least 5ma
If you have a good steady reading and the flame goes out the problme is the board
It the fire did not change at all then I would say the flame rod is bad. It may be expanding and changing its resistance.
If it fails agian i would repalce the flame rod