Guessing water heater. Does one switch feature a lightening bolt (Electric) and the other perhaps something that might be interpreted as a Flame (And that one has the fault light) (Mine say Elec and Gas)
Fault light is for gas.. If it tries to light 3 times and still does not detect the flame (NOTE: It can light and still not detect) it will fault. Or if the HIGH TEMP CUTOUT opens it will fault on both gas and electric.
Or.. .The winterize instructions say to "Remove the red wire" this is the wire that goes to the ECO thermostat (high temp cutout) and if you remove it during winterizeration and forget to restore it when you re-fill in the spring... Fault.. as well it should cause empty is not a good way to operate the water heater by accident.