Why Does Sink Back Up When Dishwasher Runs

Why Does My Sink Back Up When the Dishwasher Runs?

If your kitchen sink fills with water every time the dishwasher runs, these two appliances share a drain line — and that line has a blockage. This is a very common kitchen plumbing problem, and in most cases you can fix it yourself.

Why This Happens

Your dishwasher drains into the same drain line as your kitchen sink (usually connecting at the garbage disposal or the sink drain). When the dishwasher pumps out water, it flows into the shared drain. If there's a grease or food clog in that shared line, the dishwasher water has nowhere to go — so it backs up into the sink.

Step-by-Step Fix

Step 1: Check the garbage disposal.
If you have a garbage disposal, the dishwasher drain connects to it. Run the disposal for 30 seconds to clear any food buildup that might be blocking the connection point. Then run the dishwasher again to test.

Step 2: Check the dishwasher drain hose.
The drain hose from the dishwasher should loop up high (called a high loop) before connecting to the disposal or drain. If it sags, water can pool in the hose and cause backflow. Check under the sink and make sure the hose is looped up as high as possible.

Step 3: Plunge the sink drain.
Use a Luigi's Sink Plunger to plunge the kitchen sink drain. Block the other sink basin (if double sink) with a wet rag first. Plunge firmly 10–15 times, then run the dishwasher to test.

Step 4: Snake the kitchen drain.
If plunging doesn't work, the clog is deeper. Use a 25ft Drain Auger to reach past the P-trap and into the drain line. Rotate clockwise to break up the grease clog.

Step 5: Use a drain cleaner.
After snaking, flush the line with Green Gobbler Drain Clog Dissolver to dissolve any remaining grease buildup. Let sit overnight, flush with hot water in the morning.

Prevent It from Coming Back

• Never pour grease or oil down the kitchen drain
• Run hot water + dish soap down the drain weekly
• Use a Sani Stick monthly to prevent grease buildup
• Run the garbage disposal with cold water for 30 seconds after each use

When to Call a Plumber

If snaking and chemical treatment don't fix the backup, the clog may be further down the main drain line or there may be a venting issue. A plumber can do a camera inspection to find the exact location.

Bottom Line

Sink backing up when the dishwasher runs = shared drain line clog. Check the disposal first, then plunge, then snake. Follow up with Green Gobbler to dissolve grease residue. Prevent recurrence with weekly hot water flushes and monthly Sani Sticks.