Without a doubt, one of the most common issues I see pertains to the dishwasher installation. The issue is that the dishwasher drain line does not make a high loop underneath the kitchen sink. In the diagram below, you can see that a high loop is just what it sounds like.
As you see, the dishwasher drain should make a high loop up to the bottom side of the countertop. The high loop is necessary for 2 reasons. The first is to ensure proper drainage of waste water during the drain cycle and prevent siphoning. The second is to prevent contaminated water from the garbage disposal or sink drain from backflowing into the dishwasher.
An alternative to the high loop would be an air gap that is installed through the countertop. An air gap device is used to create a siphon break for the same purposes as the high loop. As you can see in the photo below, it is a bit unsightly. For this reason most homeowners opt for the high loop.
Although all new dishwashers come with a high loop integrated into the unit itself, every installation manual I have read requires the high loop be installed under the sink as well. Luckily this is not a hard requirement to satisfy. A simple and inexpensive fix would require a piece of metal (or plastic) strapping and a screw (left), or for a few dollars more, you could purchase a u-bend bracket (right). Both of the options should be available at your local hardware store.
Colin is a Certified Professional Home Inspector as well as a licensed MS Residential Builder. He has been remodeling homes since he was 14 and even built his own house by hand from the ground up. Colin is also the owner of Wilson Home Inspections.