There are any number of reasons why your dishwasher would not drain and here we will explain the most common causes that we find to be causing the water not to be drained from your dishwasher.
Blockages In A Dishwasher
The first and most common cause of any laundry appliance not draining the water away is of course a blockage of some kind.
In your dishwasher this will often be due to food detritus that has built up over time, often as a result of constant low temperature or short wash cycles being used that allow this to build over time and eventually block something with gunge.
If this is the cause then there's no other cure other than to strip the dishwasher down and clear it out normally as running more cycles will prove fruitless normally.
Run a hot wash at least once a week to prevent this from happening and avoid using short or eco wash programs continually
Next is the odd toothpick, bit of glass or similar that finds its way into the dishwasher drain pump and stops it from pumping out the water.
Again often the only course is to strip out the pump and clear it of any debris that has managed to get in there.
When either of these things happen we often find that the filter in the bottom of the dishwasher has become dislodged, has not been fitted correctly or is damaged allowing small items that can block your dishwasher through.
Ensure that filters are clear, undamaged and fitted correctly
The waste hose being blocked is not common but, equally not uncommon either.
This can get blocked with grease that is carried from your dishwasher where the water is hot to the end of the waste pipe where it drains into the house waste system. As the water cools there it allows the grease to solidify and congeal, blocking the drain outlet.
Many of these issues can be avoided by using full hot wash programs on your dishwasher, often marked as "heavy" wash programs regularly or, by regular use of a dishwasher cleaner such as Affresh.
Dishwasher Drain Pump Fault
Of course the other common failure is the drain pump in a dishwasher.
For almost all this is a simple electric motor that will be installed around the sump area.
You can find out much more about the drain pump in your dishwasher here but generally speaking the hardest part is reaching the pump itself as this can often be a challenge on many modern dishwashers given the limitations on space. The area where you will find them is generally very tight on space and difficult to work in whilst the actual testing of the pump and seeing if there is anything wrong with it is relatively simple.
Normally there will be only two conditions to have any concern about electrically, is the pump winding coil open circuit or not. If it is then it is faulty and should be replaced, if not then the pump motor is most probably not faulty.
The only other part of a modern dishwasher drain pump that will be faulty, especially on magnetic type pumps, is the pump impeller itself. If this is loose or damaged then it will not pump water from your dishwasher and this is usually very easy to see.
More Reasons a Dishwasher Fails To Drain
Of course there are occasionally other reasons why your dishwasher would perhaps not drain but they will be very rare exceptions indeed and will probably require that you seek a professional appliance repairer to address.
Any appliance repairer will look at the above long before looking for another explanation other than in a few model specific examples, such as dishwashers from Zanussi, AEG and a few others that use the wash pump to also drain the water from the dishwasher.