Your dishwasher is connected to the kitchen drain through an air gap. The air gap lets air into the dishwasher as it drains so that it can drain in a controlled way without siphoning out all the water or pulling wastewater into the dishwasher. A problem with the air gap can cause the dishwasher not to drain at all, or it can cause water to leak into the sink or onto the countertop. Air gap problems are usually caused by a leak or a clog in the drain hose and are easy to fix if you know what to look for.
Remove the rubber hose that runs from the air gap to the sink drain or garbage disposal. Use a screwdriver to loosen the screws that hold the clamps in place at each end of the hose, and then remove the hose.
Insert a dowel into the hose to clear any debris that might be clogging it.
Examine the place where the hose enters the garbage disposal, if you have a disposal. Garbage disposals have a knock-out plug where the dishwasher hose enters, and the dishwasher won't drain properly until the plug is removed. This may be your problem if the disposal is new. If the plug is present, hold the metal end of the screwdriver against the center of the plug and tap the screwdriver handle with the hammer until you have knocked out the plug.
Reconnect the hose to the air gap, but leave the other end of the hose free. Use a towel to dry the outside of the hose.
Put the free end of the hose in the sink and turn the dishwasher on. Wait until the dishwasher has partially filled with water, and then skip forward to the drain portion of the dishwasher's cycle. Watch the hose for leaks as the dishwasher drains. Circle any leaks you see with a permanent marker.
Press a dry paper towel to the outside of the hose to check for water, which indicates a leak. Mark any wet areas with a permanent marker.
Replace the leaky hose, or dry the outside of the hose completely and spread epoxy putty over the areas you have circled. Spread the putty so that it covers a circular area of at least a 1-inch diameter around each leak. Wait at least 24 hours for the putty to dry.
Reattach the hose and run the dishwasher.