The thermal cutoff is a one-shot diode that trips due to flare back from burner (190*F)
Heater will not light if thermal cutoff trips until replaced.
The t-stats are fixed temp set points
Normal opens at 140*F and the ECO (Energy Cut OFF) opens at 180*F
So with just tepid water either the normal t-stat is opening early OR you have cold water mixing with hot water (water heater bypass valve open, shower turn off via handle vs knobs)
Check the valving
T-stats are 'normally closed' then 'open' when set point reached.
T-stats need to be tight against tank wall
T-stats terminals can get loose/corroded inhibiting good contact
New set (normal/ECO) cost $15
There is also an 'adjustable' one that can be used in place of 'normal' t-stat (110*-150*F)
