If everything is working as it should, and your dishes are getting clean, a small amount of standing water in the dishwasher is normal and probably nothing to worry about. However, an unusual amount of water and debris usually indicates a blockage or a malfunction of the drain pump. Fixing a drain pump is a job for an appliance repair pro, but you can unclog the dishwasher yourself.
The dishwasher drainage network includes the drain pump in the appliance itself, which is protected by a filter screen, the drain hose and the sink drain. The blockage could be anywhere in this network. Some parts of it are easier to check than others, and it makes sense to check the easy parts first.
Begin at the Sink
The dishwasher drain hose connects to the garbage disposal or the sink drain tailpiece by a flexible plastic hose, and the dishwasher won't drain if there is debris in the hose or sink drain. If water is draining slowly from the sink, there's probably a blockage in the sink P-trap. Clearing that will probably take care of the excess water in the bottom of the dishwasher.
You can check for debris in the drain hose by unclipping the hose and poking around inside the sink connector with a chopstick. The hose may make a loop at the top of the sink and connect to a metal dome on the sink deck known as an air gap. Check that connection for debris as well.
Remove Debris From the Filter and Drain Hose
If you don't find any debris in the sink, turn your attention to the dishwasher itself. When you open it to look inside, check the door to make sure it's closing properly. If it doesn't close all the way, the control panel may not be signaling the drain pump to come on.
A filter basket at the bottom of the dishwasher separates the washing compartment from the drain pump. To remove this basket and clean it, you must first remove the rotary sprayer. On some models, this lifts right off, but on others, some disassembly may be required. Check your manual and follow the instructions to remove it.
Once you've exposed the filter, you can simply lift it out and clean it with a garden hose. When you remove it, you may find debris clogging the drain pump, so go ahead and remove that. If the filter and drain pump are unclogged, and you don't find debris in the filter or drain pump, the clog may be in the drain hose at the point at which it connects to the dishwasher. You'll have to pull the dishwasher out from the wall to disconnect the hose and clean it.
Can You Unclog the Dishwasher With Chemicals?
Some plumbers recommend using a homemade cleaning solution to unclog a dishwasher. Mix up a solution of baking soda and vinegar and pour it into the bottom of the dishwasher. Let it foam and bubble for 10 or 15 minutes and then pour in a pot of boiling water. It may help to run the dishwasher through a cycle while the vinegar and baking soda are still working.
Never try to unclog a dishwasher using Drano or any similarly caustic drain cleaner. The active ingredient in Drano, sodium hydroxide, generates a large amount of heat that can damage the drain hose and force you to replace it. Besides that, if you put Drano in the dishwasher, there is a risk of getting the toxic chemicals on your dishes the next time you use it.