Air gaps are a relic and not one likely to make it to the future. However, if you have an older home and the same dishwasher it came with, yours may still be using an air gap system to drain. This means that when your dishwasher isn’t draining, an air gap is just one more potential cause to check.
If you have a small nub parallel to your sink faucet, then your dishwasher uses an air gap system. It is not something that modern plumbers like to install and many will now install newer systems when installing a dishwasher. However, if your dishwasher is an old workhorse, it may still use it. In this system, the dishwasher essentially pumps the water up into the nub where it then flows into the drain system in an up and then down fashion. This is used to prevent backflow of dirty wastewater back into the dishwasher and on your clean dishes.
The real issue with this system, however, is that food particles that are common in the wastewater of a dishwasher can clog it. When this happens and the force can’t dislodge them, your dishwasher will not drain. So if you have an air gap system and your dishwasher isn’t draining, you need to check the air gap.
To check an air gap, you need to remove the cover on the top of it by your sink faucet. Now you will need to remove the diverter inside which should just have a locking clip. Now you can see inside. Be sure to remove any and all blockages in the area.
If you don’t see anything, then your investigation can move on to other potential problems that are preventing drainages, such as a faulty drainage pump or blockages deeper in the actual dishwasher system.