Check your float switch. Your thermostat is or should be wired through the float switch. So if the water level rises it engages your emergency overflow switch which will open the power circuit to the nest, hence the E3 error code. If you lift the flow switch out of where it is is the drain line, first see if there is water in the float, second look at the nest and see if it is back on line. If it is on line your drain is clogged causing the float to kill the system. It kills the system to prevent it from producing more water and overflowing into your home. What ever you do do not leave the float switch out, your thermostat will work, but if it's out your system will continue to produce water and overflow into you house. Use a wet dry vacuum to clear the drain from where it exits outside. Then as a preventative measure add Clorox to your drain to help in keeping it clear. Down side is your house is going to smell like a hospital for a bit till the smell clears out
