Could be gas side SET of t-stats
Under the rubber cover above gas valve. Right side is 12V DC set of t-stats (high temp connecting wire normal temp....RED wires)
Using a Voltmeter...check if you have 12V DC voltage at bottom spade connector when you turn GAS ON.
12V DC has to go thru top t-stat (HI) down connector wire and then thru bottom t-stat (normal) to get to circuit board.
IF water temp is 130*F then Normal t-stat will be OPEN (no DC thru it)
BUT if water temp is 100*F then Normal t-stat Closes allowing DC to circuit board.
Now if DC is going to circuit board OK then circuit board sends 12V DC to gas valve solenoids (6-8 seconds during spark) via Brown wire
Instead of throwing parts at it----Trace the DC and find where is stops.
