Integrated Drains

• Even though a dishwasher and sink have separate water supply lines, the two plumbing fixtures have integrated drain systems. The dishwasher's drain line connects to the sink's drain system at two different points. The air gap in the middle of the dishwasher's drain line connects to the bottom of the sink, near the drain. The end of the drain line connects to either the sink's drainpipes or to the side of the sink's disposal.
  
  

Dishwasher Clog

• A clog in the dishwasher's drain line may lead to the waste water from the dishwasher flowing into the sink. When a clog forms in the dishwasher's drain line near where it connects to the sink's drainpipes or the disposal, the water backs up the drain line until it reaches the air gap. The air gap stops the waste water from traveling all the way up the drain line and emptying into the dishwasher. Instead, the waste water flows out of the air gap and into the sink's drain body.
  
  

Sink Drainpipe Clog

• A clog in the sink's drainpipes may also lead to the dishwasher's waste water flowing up and into the sink. You need to use a clamp to seal off the dishwasher's drain line where it connects to the sink's drain before plunging the sink; otherwise, the water will flow up the dishwasher's drain line. If you still have problems after plunging, you must disassemble the drainpipes under the sink and feed a drain auger's flexible line into the pipe that goes through the wall.
  
  

Disposal Clog

• If the disposal or the waste pipe that connects to it develops a clog, the waste water from the dishwasher cannot exit the disposal, so it is forced to flow upward and into the sink through the drain body. Plunging may remove the clog, but you may need to remove the drainpipe that connects to the disposal. Some foods, such as carrot or potato peels, do not chop up easily in the disposal and then stick to the sides of the drainpipe. You must remove the food debris or replace the pipe with a new one.