OK..just for Clarifiation'
Thermal Cut OFF.......that is the fuse inside the clear plastic sleeve
Brown wire from circuit board to T-STAT
Brown wire from T-STAT to THERMAL FUSE back to circuit board
(thermal fuse blows---it has to be replaced. One Time Fuse)
T-stat and ECO (Energy Cut Off----HIGH Temp T-stat) wiring being reversed would NOT cause overheating of water.
T-stat (if functioning) opens at 140*F........it would STILL open at 140*F even if wiring was reversed.
Having wiring reversed would trigger Fault Light due to location of the DC being lost in the circuit (at ECO position which does trigger Fault Light)
Just for general maintenance:
Clean up connector/edge surface on circuit board
REPLACE the T-stat/ECO (clean tank surface)
Tighten ALL wire spade connectors so they fit tight on the terminals
Here is wiring diagram........