In a kitchen or bathroom sink, odor may come from the stopper, overflow passage, garbage disposal, trap, or sewer gas. Locating the exact source is more useful than pouring products down the pipe blindly.
Check the drain and overflow separately
In bathroom sinks, the overflow channel can collect biofilm and smell even when the main drain is clean. Clean it according to the fixture design.
In the kitchen, clean strainers and the disposal if present
Food caught under a splash guard or strainer can smell. Disconnect power to a garbage disposal before putting hands or tools near the grinding chamber.
Check the trap and for leaks
A trap should hold water. If it leaks, runs dry, or the odor appears after another fixture drains, a seal or venting issue may be involved.
Do not mix drain-cleaning chemicals
Bleach, acids, and commercial drain cleaners can react and release dangerous gases. If a chemical product is already in the drain, tell the plumber before the trap is opened.