A dishwasher is a convenience item that requires no attention while it operates. Some situations may cause the dishwasher waste water to back up into the kitchen sink instead of flowing out through the kitchen drain lines. An air gap on the back of the sink supplies the dishwasher drain line with air so that the water pressure will not drain the dishwasher except on the draining cycle. Clogs in the plumbing from the air gap Y or the P trap will cause the dishwasher to discharge into the sink. When a garbage disposal flow has a blockage, it can also cause dishwater to overflow into the sink.
Turn both the cold and hot water supply valves off fully in a counterclockwise direction. Turn the electricity to the kitchen circuit breaker off to avoid electrical shocks.
Remove the slip joint underneath the air gap under the sink cabinet by turning counterclockwise with adjustable pliers. This is a PVC fitting in the shape of an upside down Y that connects the air gap, dishwasher drain line and garbage disposal together. Remove the slip joint on the dishwasher and garbage disposal sides in the same manner.
Remove the P trap from the garbage disposal by turning the screw on the metal band counterclockwise with a flat screwdriver to loosen and pull the P trap loose. Remove the other end of the P trap that connects to the exiting drain line by turning the slip joint counterclockwise. Remove the P trap.
Observe in the insides of the P trap and the Y fitting to see if there are any obstructions or clogs. Take both fittings outside and wash with a garden hose with high water pressure to make sure that any obstruction will exit the fittings.
Shine a flashlight into the garbage disposal opening that leads to the air gap and dishwasher drain line to see if there is a large item blocking the opening. Remove any items that you see covering the hole.
Reattach the P trap to the garbage disposal by turning the screw on the metal band clockwise to tighten the junction. Replace the P trap on the exiting drain line and the Y fittings by placing the fittings together in the correct places and turning the slip joints clockwise with adjustable pliers until each is tight.
Turn the water supply lines and the circuit breaker back on. Test the system to see if the dishwasher is operating properly by running the appliance through a cycle.