The smell from the sink drain is living under the cabinet, not in it: the trap clean

The smell that hits the kitchen when the wind turns, the faint rot the sponge never explains, the bathroom odour the family attributes to everything except the plumbing - nine times in ten it is coming out of the trap, the little loop of pipe under every sink whose entire job is to hold a cup of water as a plug against the drain's air. That plug gets dirty, the cup gets a lid of gunk over a sludge of grease, and the smell negotiates around it into the room. The fix is the most underrated five minutes in the house: a bucket, a twist, and the knowledge of which way the slip-nuts turn.

The trap, in the one paragraph that makes the rest obvious

Water used in the sink flows out through a bent pipe - the U or bottle loop - and the bend always keeps some water in it: that standing cup is the seal between the house and the drainage system's air, sewer gas included. Everything the trap is, everything it fails at, follows from 'a cup of held water with sink gunk flowing through it': grease from the dishes coats it, the soap and food bits build the silt in the bottle's bottom, and the film's own biology is the smell. A trap that has been cleaned looks new, drains fast and offends nobody; a trap nobody has opened in two years is the household's own compost experiment, wired to the room.

The five-minute clean, with the two hardware truths

1. The bucket and the towel first: the trap holds a cup of the sink's oldest water - a shallow tray under the joints and an old cloth for the spill ends the mess before it starts. The gloves the roach page keeps insisting on apply here with more reason.
2. Slip-nuts, by hand: modern traps are held together by slip-nuts - the smooth rings that unscrew without a tool on 90 percent of installations (cloth and gentle pliers if decades of limescale disagree) - and the hardware truth number one: the washers inside the joints go back the same way they came out, seated flat, or the 'fix' becomes a leak. Unscrew the trap's bottom (bottle types have a cleaning screw at the base - the design's whole purpose) or take the U out whole.
3. Out, over the bin, into the tap: the gunk goes in the bin, the piece gets scrubbed under running water with the brush - and the pipe above, into the wall, gets the flashlight look and the wire-or-brush pass while you are standing there, because the trap's mouth is where half the household's lost jewellery and every cotton-bud colony was last seen.
4. Reassemble, then the slow-fill test: run the sink full, pull the plug, and watch the joints with the cloth dry - a drip that starts at the washers is the twenty-second re-seat, not a re-do; the desk has watched households call the plumber for a flat washer with this minute of patience skipped.

The dry trap: the smell with no gunk

The second trap smell has no dirt in it at all - the guest bath, the floor drain in the spare room, the yard channel's indoor tail: a trap that is not used, evaporates. The cup dries out, the seal lifts, and the drain's air walks into the house through an empty, perfectly clean pipe. The smell that 'came from nowhere' after months of an unvisited room is this line, and the fixes are a pitcher of water down (the re-seal is instant and the smell leaves with the evening) and the calendar habit for the drains the house forgets - a cup monthly in the guest bath, the unused floor trap, the yard gully's tail. The professional's version for the drains that dry every season: the trap primer and the one-way membranes the trade sells as odour-stoppers - honest products, installed as the plumber's job, not this page's.

The smells that are not the trap, named so the household stops cleaning the wrong part

• The rotten-egg note from the water itself, not the drain: that is the borehole's sulphur chemistry - the smell test of the taste-and-odour guide settles it in one sniff of a glass carried away from the sink.
• The gurgling in the trap when another fixture drains, or the slow emptying that burps: the system is pulling its own seal - the block or the vent is downstream, the soakaway guide's night-gurgle paragraph is the same symptom with the bigger stakes, and the trap under the sink is innocent.
• The musty cup-of-grease smell with a slow drain, where the kitchen's oil habit is the confession: the trap's grease is what the yard's grease pit exists to catch (the guide beside this one), and a household that pours oil down the daily drain is writing its own trap page into the next year's plumber invoice.

The desk's standing rule

The sink that smells gets the bucket-and-twist the same evening - five minutes, two hands, the washers' memory - and the unused drain gets its cup of water monthly for life. The trap is a seal made of nothing but held water and good will; keep it wet, keep it clean, and the drain's smell stays where it belongs, which is nowhere the nose can reach.