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Yes, a dishwasher needs an air gap to prevent backflow. Additionally, in many locations, air gaps or another measure to prevent backflow are required for building code compliance. There are alternatives to an air gap, like a Studor vent or a loop and trap design, but an air gap is the simplest and most popular solution.  

A clogged air gap can cause a dishwasher not to drain. There are, however, other potential issues that can lead to improper drainage. For example, if you have attached the air gap to a garbage disposal and forgotten to remove the knockout plug, wastewater may backflow into the dishwasher. Check that the air gap is installed properly, and then look for clogs in the air gap and dishwasher filters. 

When deciding whether to repair or replace your dishwasher, consider its age, the repair cost, and its warranty status. It is often worth repairing a dishwasher if it is only a few years old, the problem is minor, or the appliance is still under warranty. A common guideline is the 50% rule: if the repair cost is less than 50% of the price of a new unit, a repair is generally recommended. However, if the dishwasher is more than 10 years old or requires frequent repairs, replacement is likely the more cost-effective choice to avoid future issues. For context, a new dishwasher costs between $1,000 and $1,500 on average, while typical repairs cost several hundred dollars. Simple fixes like a damaged gasket or clogged filter can be inexpensive, but major repairs like a new pump or motor can be costly.

If your dishwasher isn’t draining, some common causes include a clogged drain filter, drain hose, or air gap. Also, if you have a garbage disposal, a clog in the disposal can back up your dishwasher since they share plumbing, as can a clog in your kitchen sink’s plumbing. A handy DIYer may be able to figure out how to unclog a dishwasher, but you may need to call in a plumber if you can’t locate and clear the clog yourself.