Bad Smells From Your Drains: Causes and Fixes
Dry traps, failed seals, broken vents, finding the source instead of masking it.
The usual suspect: a dry trap
Every fixture holds a water plug in its trap that blocks sewer gas. A shower unused for a fortnight, a washing machine standpipe, a floor gully, any of them can evaporate dry and let the smell in. Run water in every fixture for thirty seconds; if the smell fades in a day, that was it.
Failed seals and vents
A smell that returns when it rains points at a damaged seal or a blocked soil vent pushing gas back through the weakest trap. The wax seal under a toilet, a cracked collar on the stack, or a bird's nest in the vent are the classics. These need finding, not Febreze.
When it's the drain itself
A persistent smell plus slow drainage plus lush patches in the lawn is the signature of a cracked or collapsed run leaking into the ground. A CCTV survey settles it in an hour and gives you the exact position and depth, which is the difference between a patch repair and digging up half the garden.