The short answer
A P-trap holds water to block sewer gas, but a sink odor is not automatically a dry trap. First determine whether the smell comes from the drain opening, the overflow passage, residue on the stopper, a leaking joint, or another fixture.
Separate the common causes
Clean removable stopper parts with the method allowed by the fixture manufacturer. With the cabinet dry, check the trap and slip joints for moisture. Run water briefly and listen for gurgling.
| Observation | Likely next check |
|---|---|
| Odor follows long non-use and fades after water runs | Evaporated trap seal |
| Odor remains at the stopper or overflow | Biofilm or trapped debris |
| Cabinet is damp | Trap or drain-joint leak |
| Trap empties or sink gurgles | Venting, siphoning, or downstream restriction |
| Several fixtures smell | Shared drainage or vent issue |
Oatey explains that the bend of a P-trap retains a water seal. Its installation guidance also shows that slip-joint washers, alignment, and the direction of the trap assembly matter. A misaligned trap can leak even when the nuts feel tight.
Safe actions
For an infrequently used sink, running water can restore the seal. Clean accessible stopper and overflow surfaces without mixing chemicals. If a slip joint is wet, shut off faucet use, place a tray below it, and inspect the washer and alignment according to the part instructions.
Do not convert the assembly to an S-trap, add improvised flexible loops, or block a vent. Do not keep tightening a cracked or cross-threaded plastic nut.
Call a plumber when
Get help if odor persists with a full trap, the trap repeatedly siphons dry, multiple fixtures gurgle, wastewater backs up, or concealed piping is involved. Sewer-gas odor plus headache or nausea is a reason to leave the area and obtain prompt help.