I have had my ATO for about 6 months and I have been having trouble with the switch sticking off and on now for about the last 2 months or so. I just recently mentioned it to the store I bought it from and I just brought it back to them today and they traded it in for a brand new system. Might just ask the store if they can trade it out for you???
It was the sensor in my sump that would stick also. It had gotten worse over the last couple of weeks, to the point that in order for it to fill ever, I had to go in and manually move the switch.
I havent dosed with kalk in that particular tank, so not sure what caused the problem. Hopefully this new unit will work just fine! I am waiting to reinstall it until I get my new tank switched over, hopefully this week.