Hi Andy here is what Roger Vitko had to say (If it was working before, the issue is likely a bad pump. If this is a new install, we need to look at other possibilities.)
Looks like I will have to buy a new Tunze Nano Osmolator.
I have another question on the ATO.
This morning when check my tank I noticed the water leave was low.
The ATO is not coming on. When I unplug the unit and plug it back in it only runs for
a couple of seconds then stops. I removed the censor and tried moving the float up and down
nothing, it won't start the pump. I am lost, any ideas???
Sorry for being a pest.