Odor after plumbing repair

Under-sink plumbing after repair where a new odor started.

Quick Answer:

If you notice a new sewer- or rotten-egg smell right after a sink repair, start with simple checks under the cabinet: look at every slip-joint nut and washer for alignment, confirm the trap arm slopes slightly toward the wall, and sniff around the air admittance valve (AAV) and all connections. Many odor problems come from a misaligned washer or a trap that wasn’t reseated fully — these can let gas escape without dripping water.

Why This Happens

After working under a sink, small parts can shift or seating can change. Common causes of a new odor include:

  • Washer or slip-joint nut not seated correctly, allowing sewer gases to pass without an obvious leak.
  • Trap arm installed flat or pitched the wrong way so water doesn’t hold in the trap as intended.
  • A loose or failed connection at the disposal or tailpiece where gas can escape but water may not drip.
  • An AAV that is stuck open or installed in a place where it draws air from under the cabinet.

Step-by-Step What to Do

1. Clear the area and look closely

  • Turn on the cabinet light or use a flashlight. Remove stored items so you can see all fittings.
  • Check each slip-joint nut visually for gaps and the washer position. The washer should sit flat and centered inside the nut, not folded or twisted.

2. Inspect and re-seat washers and nuts

  • Hand-loosen and re-seat any washer that looks misaligned. Push it into place before re-tightening the nut by hand.
  • Don’t assume a dry joint is sealed; sewer gas can escape around a warped washer without dripping water.

3. Confirm trap arm slope

  • Look along the trap arm toward the wall. There should be a slight downward pitch from the trap to the wall (roughly 1/4″ per foot), not a flat or uphill run.
  • If the trap arm is flat or pitched wrong, loosen the slip nuts, realign the pipe with a slight slope toward the wall, and retighten gently.

4. Sniff key points

  • Smell around the AAV (if present), the trap inlet, disposal connection, and tailpiece. The source is often close to where the smell is strongest.
  • If you suspect the AAV, temporarily crack open a cabinet door and listen/smell while running water to see if the odor changes.

5. Test and verify

  • Run water and the disposal (if present) for a minute, then close the cabinet and check again. If the smell appears only with the disposal or when water runs, that points to specific disposal or vent issues; see the related posts like Smell only when disposal runs.
  • If the smell goes away after reseating washers and correcting the slope, you likely found and fixed the problem.

What Not to Do

  • Don’t overtighten plastic slip nuts after a repair—warped washers can actually leak odor.
  • Don’t ignore a persistent or strong sewer smell; if you can’t get a good seal or the smell is intense, a plumber is appropriate.
  • Don’t use flammable sealants or improvised plugs under a sink where an AAV or vent may be present.

When to Call a Professional

  • If you can’t find the source after the checks above or the smell is very strong, call a licensed plumber. Persistent sewer gas can indicate a venting problem, a cracked trap, or a hidden leak.
  • If tightening or replacing washers doesn’t stop the odor, a plumber can pressure-test the drain, inspect vents, and replace parts safely.

Safety Notes

  • Sewer gas can contain hydrogen sulfide and other unpleasant gases. If the smell makes you lightheaded or sick, leave the area and get fresh air immediately.
  • Turn off electricity to the disposal before working on its connections. Use gloves and eye protection when handling drains or cleaning parts.

Common Homeowner Questions

  • Why does the smell come and go?
    Temperature, water use, and whether the disposal runs can change pressure and venting, so odors may be intermittent.
  • Should I replace a washer if it looks slightly warped?
    Yes—replace a warped or cracked washer. A new washer is inexpensive and often fixes odor problems that don’t show a water leak.
  • Could cold weather make the odor worse?
    Yes; cold weather can change house pressure and vent performance — homeowners often see seasonal differences in smells, including moments when Odor worse in winter.

For more related articles, see the Food Waste Odors & Bio-Buildup hub.

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