Quick Answer:
If the kitchen sink stays normal while you run the tap but suddenly backs up when the dishwasher pumps out, the most likely problem is a partial restriction in the downstream branch that carries dishwasher flow. Watch the exact pump-out moment, then inspect the branch from the sink/disposal to the main drain and the dishwasher connection for a clog or a wrong slope.
Why This Happens
- Dishwashers pump a concentrated burst of water and waste into the drain. If the downstream pipe is partly blocked, that pulse can overwhelm the reduced capacity and push water back into the sink quickly.
- Common causes include buildup at the tee where the dishwasher ties into the sink line, a clogged section of the branch line, a full or misaligned p-trap, or a partially closed cleanout downstream.
- Venting problems and air locks can make a drain accept water slowly until a pressure change from the dishwasher forces a backup.
Step-by-Step What to Do
1. Observe the pump-out moment
Run a normal dishwasher cycle with the kitchen sink empty and watch closely as the dishwasher reaches its drain phase. Note whether the sink water rises the instant the dishwasher pump starts or only after a short delay. This tells you if the problem happens only during that pulse.
2. Check the garbage disposal and sink trap
- Turn off power to the disposal and remove the trap. Clear any obvious debris.
- Make sure the dishwasher hose connection at the disposal or sink tailpiece is secure and not clogged.
3. Inspect the downstream branch for restriction
- Remove the trap and snake a short section (3–6 feet) down the branch that serves the sink/dishwasher. A hand auger or small powered snake often finds buildup at the branch tee or the first elbow.
- If the snake meets resistance and you pull out grease or scale, clean thoroughly and test by running the dishwasher drain again.
4. Check the dishwasher drain hose and air gap
- Look for kinks, crushed sections, or a blocked air gap (if present) that could trap water until the pump forces it through.
- Disconnect the hose at the disposal or tailpiece and run a short test: pour water down the hose to see if it flows freely into a bucket.
5. Test for main-line or vent issues
- If the branch clears but the sink still backs up only during dishwasher drain, the problem may be farther downstream or a venting issue. Run water in the sink while someone else runs the dishwasher to confirm the timing.
- Look for slow drains elsewhere in the house; if present, suspect a main-line restriction.
6. Re-test and confirm
- After clearing and reassembling, run the dishwasher and watch the exact pump-out moment again. If the sink no longer rises, the restriction was likely in the branch.
- If the behavior changes after recent work or a service interruption, see related notes like Sink backs up after installing dishwasher or Backup after city outage for context.
What Not to Do
- Do not assume the dishwasher is broken if the sink drains poorly under normal faucet use too. A general slow drain points to a larger blockage or vent problem, not just the appliance.
- Do not pour large amounts of chemical drain cleaner into a kitchen drain where grease and plastic fittings are present; it can damage pipes and fixtures and create a hazardous mix if other cleaners are present.
- Avoid forcing tools deep into the plumbing without knowing the layout; you can damage traps, fittings, or the dishwasher hose connection.
When to Call a Professional
- If you cannot clear the restriction with a hand auger or the clog is deep in the main line.
- If multiple fixtures drain slowly or you detect sewer odors — signs of a main-line or sewer problem.
- When you’re uncomfortable disconnecting the disposal or dishwasher hose, or if electrical components are involved.
Safety Notes
- Turn off electrical power to the disposal and dishwasher before working near them.
- Wear gloves and eye protection when handling debris or using a snake; pipes can contain sharp objects and contaminated water.
- Avoid mixing chemical cleaners and use them sparingly; if you’ve used chemicals recently, flush the line well before putting your hands or tools into the trap.
Common Homeowner Questions
- Q: Why does the sink only back up during the dishwasher cycle?
A: The dishwasher produces a high-volume pump-out that can overwhelm a partial restriction in the branch line, causing a rapid backup that doesn’t occur with slower faucet flow. - Q: Can I fix this without unplugging the dishwasher?
A: You can inspect the trap and branch while the appliance is off, but disconnecting the dishwasher hose for testing is often needed to confirm flow; power should remain off for safety. - Q: Will a plumber camera be necessary?
A: If a hand snake can’t reach the restriction or the problem recurs, a camera inspection helps find blockages, collapsed pipe, or improper connections further downstream.