The most common causes of a dishwasher not draining are clogs or problems somewhere in the drainage line. This includes the dishwasher's drain hose, the air gap, garbage disposer and/or the sink drain. Screens or filters inside the dishwasher can also become clogged enough to hinder drainage. All of these problems are easy to check for, and you should always rule them out before calling in a pro for repairs. If the kitchen sink also doesn't drain well, there's likely a clog (or venting problem) in the sink's drain line, and the dishwasher will return to normal once the drain is cleared.
The drain hose is a small plastic or rubber tube (about 1 inch in diameter) that runs from the underside of the dishwasher and up through the cabinet(s) and connects to an air gap device (see below) to the garbage disposer or directly to the sink drain line. Visually inspect the hose to check for kinks or pinched areas. Note: If you've run the tests explained below and still suspect a blocked hose, you can remove the hose from the dishwasher (it's connected to the pump assembly, behind the kickplate under the dishwasher door) and clean or replace the hose.
If you have an air gap installed on the little shelf at the back of your sink, remove the air gap's cover to check for blockage inside the device; that could be why your dishwasher is not draining. Also remove the ends of the drain hoses from the air gap nipples below the sink (one is the dishwasher drain hose; the other connects the air gap to the disposer or sink drain). Clean the hose ends if they are blocked.
To rule out one of the simplest causes of a dishwasher not draining, run the disposer to make sure it's not full of food scraps. Next, remove the dishwasher drain hose from the nipple on the disposer and remove any blockage there. If the disposer was installed very recently (or about the time the dishwasher started acting up), it's possible that the installer forgot to punch out the knockout plug inside the drain nipple. You can do this yourself with a hammer and screwdriver, following the manufacturer's instructions.
Filters and Screens
Most dishwashers have a strainer screen or "glass trap" located in the bottom center of the tub, underneath the lower spray arm. Remove any debris that may be clogging the screen and, if necessary, remove the screen and clean it as directed by the manufacturer. Also clean any additional filters as applicable, as blocked filters may lead to your dishwasher not draining efficiently.
Things a Pro Might Check For
There are several mechanical parts that can fail or malfunction and result in a dishwasher not draining properly. A plumber or repair technician can run a series of tests to isolate the problem. Chances are, they'll check the pump assembly (and drive belt, if the dishwasher has one), timer, motor and drain valve (on models with single-direction motors). Often, the offending part can be swapped out with little or no collateral work or replacement required.