Quick Answer:
If the sink started backing up right after the dishwasher installation, most often the drain tie-in or hose routing is wrong or there’s a clog at the connection. First verify the dishwasher hose is tied into the correct point (tailpiece or disposal inlet) and that the hose makes a high rise before it drops to the drain. If you want more context on related causes, see Backup after plumbing repair.
Why This Happens
- Dishwasher drain water follows the path of least resistance. If the tie-in is below the trap outlet or the hose sits low, sink water can flow back into the dishwasher or cause the sink to fill.
- A low or improperly routed hose can allow sink water to run into the dishwasher instead of draining away.
- Clogs in the trap, disposal inlet or tailpiece will back pressure into the sink when the dishwasher drains. Also check the dishwasher filter—blockage there can feed water back into the sink; see Sink backs up when dishwasher filter clogged.
Step-by-Step What to Do
1. Turn off power and prepare
- Switch off the dishwasher power at the appliance or breaker and turn off power to the garbage disposal if you will inspect it.
- Place a bucket and towels under the sink before disconnecting any hoses or removing the trap.
2. Check the tie-in point
- Locate where the dishwasher drain hose connects:
- If it is tied into the sink tailpiece, the branch fitting should be above the trap outlet and above the trap’s highest water line.
- If it is tied into the garbage disposal, the hose should connect to the inlet on the side of the disposal where the dishwasher hose fits and is clamped securely.
- Look for loose clamps, a hose pushed in too far, or a hose clamped into the trap rather than a proper branch or disposal inlet.
3. Inspect hose routing and the high loop/air gap
- The hose must rise up high under the counter (a “high loop”) before it drops to the drain. This prevents sink water from flowing back into the dishwasher.
- If your setup uses an air gap device, make sure the air gap is installed and not clogged.
- Re-route and secure the hose so it reaches as high as possible under the countertop and then drops to the inlet.
4. Check for clogs at the trap, tailpiece and disposal
- Run water in the sink and watch the drain: if it’s slow or backed up, remove the trap and clear any debris.
- If you have a disposal, run it briefly (with power on and clear instructions) to clear food solids; if you remove the disposal’s knockout plug later, ensure the power is off while doing that work.
- Reconnect parts and run a short dishwasher drain cycle while watching for backups.
5. Test the system
- With the hose secured in the correct place and any clogs cleared, run a rinse or drain-only cycle and watch the sink. If the sink no longer fills, the issue was likely routing or a clog.
- If the sink still backs up, you may need to isolate the problem by disconnecting the dishwasher drain hose at the tie-in (catch water in a bucket) to see whether the sink drains normally without dishwasher discharge.
What Not to Do
- Do not connect the dishwasher drain hose low into the trap or below the trap outlet. That allows sink water to enter the dishwasher.
- Do not leave the hose lying down behind the disposal or sink drain where water can pool; the hose must rise to form a high loop or go to an air gap.
- If you’re unsure about a code-compliant setup or how to remove a disposal knockout safely, professional help is appropriate. Don’t guess at code-required parts like air gaps in areas that require them.
When to Call a Professional
- The sink still backs up after you’ve confirmed the tie-in and fixed hose routing.
- There is persistent clogging that you cannot clear from the trap or disposal, or sewage odor is present.
- You need a permanent code-compliant fix (air gap installation, new tailpiece, or disposal work) and prefer a plumber to ensure it is done correctly and safely.
Safety Notes
- Turn off power to the dishwasher and garbage disposal before working on electrical or moving parts.
- Use gloves and eye protection when handling drain debris. Have a bucket and rags ready for water when you disconnect hoses.
- Do not use heat or hammering to force a hose or fitting; if parts are stuck, call a professional to avoid damage or injury.
Common Homeowner Questions
- Why does the sink gurgle when the dishwasher drains?
Air is being pulled through the trap or there’s a partial clog; check the high loop and clear the trap. - Can I tie the dishwasher directly into the garbage disposal?
Yes, if the disposal inlet is used and the knockout has been properly removed and the hose clamped securely. - Is a high loop the same as an air gap?
No. A high loop is a simple hose routing; an air gap is a code-required device in some areas that prevents backflow and must be installed above the sink.