Quick Answer:
If your kitchen sink suddenly backs up seconds after you turn the faucet on, first check whether other fixtures are affected. If only this sink is slow or backing up, the problem is likely at the under-sink trap or the kitchen branch. If toilets or tubs are slow too, suspect a larger branch or main line problem. Start by isolating the sink, remove the trap cleanout, look for a solid plug of debris, and retest flow into a bucket before reassembling.
Why This Happens
Sudden backups are usually caused by a blockage that lets water flow until a pocket behind it fills and then forces water back up the nearest open drain. Common block sources in a kitchen branch are:
- Grease and food that build a soft plug that collapses and then re-forms.
- Small objects or fibrous material collecting at the trap or its cleanout.
- A downstream clog in the branch line that suddenly causes the trap to overflow when you add more water.
Check other fixtures first to tell whether the issue is local to the sink or part of a larger plumbing line problem. If you were just cooking and the sink backed up, you may also want to read about Overflow during heavy cooking. If the backup followed recent work, see how that can relate to Sink floods after plumbing repair.
Step-by-Step What to Do
1. Check other fixtures
Turn on a nearby bathroom sink, run the tub, and flush a toilet. You’re looking for slow drainage or backups.
- If toilets and tubs are also slow or backing up, this suggests a larger branch or main sewer issue. Stop here and consider calling a plumber (see When to Call a Professional).
- If those fixtures drain normally, the problem is most likely under the kitchen sink or the immediate branch line.
2. Prepare and protect the area
- Put on gloves and eye protection.
- Place a bucket and towels under the trap to catch water and debris.
- Have adjustable pliers or a wrench ready, and a small brush or rag for cleaning.
3. Remove the trap cleanout / P-trap
Loosen the slip nuts holding the trap and remove it carefully. If your setup has a trap cleanout plug instead, remove that plug. Expect water and trapped debris.
- Look inside: you may find a solid plug of grease, compacted food, or foreign objects.
- Clear the plug and rinse the trap parts. Use a small brush or rag rather than your bare hands.
4. Retest flow into a bucket
Before reattaching everything, point the trap outlet or pipe into your bucket and run the faucet briefly.
- If water flows freely into the bucket, the immediate blockage is cleared — reassemble the trap, check for leaks, and test the drain again.
- If water still backs up or barely flows into the bucket, the clog is downstream of the trap and likely requires a plumber or a drain snake to reach it.
5. Clean up and inspect
- Reinstall the trap carefully and hand-tighten the slip nuts, then check for leaks.
- Run water and watch for a return of the backup or slow drainage.
- If the problem recurs quickly, document when it happens and what you found — this helps a pro diagnose the issue.
What Not to Do
- Don’t dump chemical drain openers into a suddenly backed-up sink — standing chemicals are a hazard to you and can damage pipes.
- Don’t keep running water continuously while the sink is backing up; that increases the risk of overflow and mess.
- If the clog does not clear at the trap or cleanout, do not keep forcing the issue yourself — a plumber is appropriate for downstream or stubborn blockages.
When to Call a Professional
- Multiple fixtures are slow or backing up (toilet, tub, bathroom sink) — this points to a main branch or sewer problem.
- The trap cleanout didn’t clear the blockage and water still refuses to pass into a bucket.
- You uncover damaged, corroded, or leaking pipes while working under the sink.
- The clog keeps returning soon after cleaning, or you can’t access the blockage safely.
A qualified plumber has the tools and experience (longer snakes, camera inspection) to find and clear blockages safely.
Safety Notes
- Wear gloves and eye protection when opening traps; wastewater can contain bacteria and sharp objects.
- Have a bucket and absorbent towels ready to control spills.
- Avoid mixing chemicals with mechanical clearing methods — drain cleaners left in pipes can injure you and the plumber.
- Dispose of solid debris and greasy buildup in a trash bag; never pour grease down the drain again.
Common Homeowner Questions
- Why did this happen so suddenly?
A soft or partial plug can hold until enough flow or pressure shifts it, then it collapses and forces water back up the nearest drain. - Can I use a plunger first?
Yes — a sink plunger can help for minor clogs, but don’t rely on it if the sink keeps backing up or other fixtures are involved. - How long should I try before calling a plumber?
If clearing the trap doesn’t fix it or multiple fixtures are affected, call a plumber rather than continuing DIY attempts.