Per Atwood Manual:
THERMAL CUT-OFF DEVICE
Current Atwood direct ignition water heaters are equipped with a thermal cut-off device. This device is located on the incoming power wire and is connected to the thermostat. The thermal cut-off is designed to permanently break circuit and shut down the water heater before excessive heat can cause damage due to obstructions in the main burner tube or flue tube caused by spiders or mud wasps. These obstructions can cause the main burner flame to burn outside the main burner tube. When the flame or the heat from the flame contacts the thermal cut-off, the circuit will open.
Thermal Cut-Off-----heat sensing diode that cuts power to circuit board if a flame backs out of the burner tube or flue tube normally caused by an obstruction in these areas.
Thermal Cut-Off
A one shot heat sensing fuse that’s normally closed and sends power to the thermostat.
When tripped by excessive heat (190°F), (i.e. blocked burner or flue tube) it cuts power to the circuit board and shuts down ignition
As suggested.....
It can be removed from system to TEST operation of WH.
IF all other components are working correctly WH will function....t-sat/ECO/circuit board etc.
Just like a fuse.......one shot, no reset. Replace if blown.