if you ATO uses a little pump (or 50 ml doser from BRS), I would put it on plug #1 or #4. I can elaborate on the electronics reasoning why if you want....or just look around and you will see why. The lower power pumps/motors sometimes will not completely turn off when used on plugs #2 and #3, but #1 and #4 are mechancically switched off. I use a BRS 50ml doser for my ATO and had this issue.