quote;
1 cause likely is t'stat
2 cause likely is t'stat or element
3 cause likely is t'stat
1. is MORE often a bad element than a thermostat
2. is a result of the red ECO button popping out because the water was too hot. See #1.
3. Only time will tell, since it takes a day or so for the symptoms to reappear.
I have replaces THOUSANDS of elements, but only a few dozen, and maybe even fewer, thermostats over the past 60+ years. Very few thermostats malfunction and then start working properly so it only takes a few minutes to determine if it, or they, is the problem, IF the heater is hot at the time. If it is cold then it takes longer because you have to wait until it gets hot enough to actuate both thermostats.