Things You'll Need
Some Oaty outlet boxes are designed with knockout plugs that won't get stuck in the drain trap, but it's best to avoid dropping the knockout plug into the drain. It could get stuck where there's existing blockage or a tight bend or other snagging point in the drain line.
Make sure there's a drain hooked up to the outlet box in the location where you remove the knockout. If there are any additional open knockouts that are not connected to drains, cover them with tape and a note reading "No Drain" to prevent someone from fitting the drain hose into the knockout.
Properly configuring the hookups for your washing machine is vital to avoiding leaks. Most of the outlets on Oatey's outlet boxes come with plastic pieces you have to remove so that pipes can be hooked up. If your Oatey washing machine outlet box has yet to be installed, clamp it down onto a workbench or have a friend hold it, as you will need both hands free and don't want the box to move around. If the outlet is already installed, it should be mounted securely enough that you can remove the plastic piece without assistance.
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Turn off cold and hot water to your washing machine as a preventative measure before proceeding.
Line up the tip of a flat-head screwdriver with the edge of the circular plastic piece you want to remove from the Oatey washing machine outlet box.
Hammer lightly on the butt of the screwdriver and move slowly around the edge of the plastic piece to break the seal. Stop when you've broken the seal approximately three quarters of the way around the circle. Do not break the seal all the way around the plastic piece, or it will fall into the drain.
Insert the tip of the screwdriver into the perforation you've made and gently pry up the plastic piece.
Clasp the edge of the plastic piece with a pair of pliers and gently twist and lift until the piece snaps free of the washing machine outlet box.