Quick Answer:

If you see water coming back out of the overflow slot while the bowl drains, it usually means the drain path past the overflow is partly blocked or the trap is filling and forcing water back up the channel. A simple test — drain a full bowl and watch the overflow slot for surging — helps confirm a downstream restriction.

Why This Happens

• Downstream clog: Hair, soap scum, mineral buildup or grease farther down the drain can slow flow. When the main drain can’t accept the water fast enough, some returns up the overflow channel.
• Overflow channel restriction: The small channel behind the overflow opening can collect debris. If it’s clogged, water has nowhere to go except back out of the slot — this is often the culprit and matches cases listed under Overflow channel blocked by buildup.
• Recent work or changed parts: If you recently worked on the faucet or drain, seals or connections can be misaligned, leading to unusual flow paths. That situation is worth checking and is related to reports like Overflow leaking after faucet install.
• Trap or vent problems: A full or partially blocked P‑trap, or a poorly vented waste line, can make the sink drain slowly and push water into the overflow.

Step-by-Step What to Do

Step 1 — Stop using the fixture

Don’t run additional water. Reducing new water into the system keeps the situation from getting worse and makes testing easier.

Step 2 — Do the full-bowl drain test

Fill the bowl to a normal level, then remove the stopper so the bowl drains. Drain a full bowl and watch the overflow slot for surging. If you see noticeable surging or backflow there while the main drain runs, that points to a downstream restriction or slow trap.

Step 3 — Clear the overflow channel gently

• Use a soft brush, a flexible bottle brush, or a long cotton swab to reach the overflow opening and remove obvious gunk.
• Flush with warm water by pouring a steady stream into the overflow opening to help wash loose debris toward the drain.

Step 4 — Check the P‑trap and tailpiece

• Place a bucket under the trap, loosen the slip nuts, and remove the trap to look for hair and debris. Clean and reinstall carefully.
• If the trap is clear but the problem remains, the clog may be deeper in the drain or the venting system.

Step 5 — Re-test and observe

After cleaning, run the drain test again. If surging is reduced or disappears, the cleaning likely fixed it. If the overflow still backs out, move to the next steps (call a pro or consider a deeper inspection).

What Not to Do

• Do not snake aggressively through the overflow opening; it can dislodge connections behind the sink.
• Don’t pour strong chemical drain cleaners into the overflow or down the drain — they can damage finishes and plumbing and create unsafe fumes.
• Avoid forcing tools or long hard rods into the channel; you can scratch or crack the overflow interior or knock loose seals.

When to Call a Professional

• Water continues to back out of the overflow after you’ve cleaned the visible parts and cleared the trap.
• There’s any sign of a leak, water damage, or you smell sewage.
• You’re uncomfortable disassembling the trap or the clog seems to be deep in the building drain or vent system.

Safety Notes

• Wear gloves and eye protection when working under the sink. Used drain water can contain bacteria.
• If you use tools, catch spilled water with a bucket and clean up promptly to avoid slip hazards or water damage.
• Avoid mixing chemical products. If chemicals have already been used, tell the professional before they start work.

Common Homeowner Questions

• Why does clearing the trap sometimes fix the overflow? — Clearing the trap removes the immediate blockage that was slowing flow and forcing water into the overflow channel.
• Can I use vinegar and baking soda on this? — Mild DIY cleaning like baking soda and vinegar can help loosen buildup, but avoid excessive use and don’t mix with commercial cleaners.
• Is this an emergency? — Not usually, but act promptly. If water is spilling or you smell sewage, treat it as urgent and call a pro.