Atwood G6A-7E uses a simple DC circuit
12V DC from On/Off switch to a 'Thermal Fuse' then to the T-stat and then to the circuit board (BROWN WIRES)
12V DC from circuit board to ECO to gas valve solenoids to ground (RED Wires)
At SAME time DC is going to gas valve solenoids the circuit board energizes the spark electrode with high voltage to cause a spark
Using a Voltmeter set to DC follow the 12V DC from switch ---thermal fuse---t-stat---circuit bd----eco---gas valve
Bad thermal fuse......no DC to rest of circuits
DC ok to circuit bd (brown wires) but nothing out to ECO/Gas Valve---bad circuit board
Always check all ground connections clean/tight
Thermal Fuse (clear tube) one shot device...blows if exposed to 190*F from flame blow-back out of combustion chamber
T-stat Opens at 140*F/Closes when water temp drops below 110*F
ECO (Energy Cut Off---High temp t-stat) Opens at 180*F/Closes when water temp drops below 150*F