Assume gas until you know otherwise. A gas leak smell is usually strong, steady, and strongest near the stove, water heater, furnace, or gas meter. Sewer gas tends to come and go and sits near drains, toilets, or an unused bathroom. If you are unsure, leave the house and call 911 or your gas company from outside.
The two smell alike for a reason. Gas utilities add a sulfur-based odorant to natural gas so people notice a leak. Sewer gas carries hydrogen sulfide, another sulfur compound. Both read as skunk or rotten eggs to most noses. What differs is where the smell comes from and how it behaves over time.
Clues for sewer gas: the smell shows up after a long absence, grows near one drain, or fades once you refill floor drains and unused showers. Clues for gas: a hissing sound near a gas line, dead grass over a buried line, or a smell that stays the same everywhere.
Never hunt for the source with a lighter or a candle, and do not wait to see if the smell passes. Gas utilities respond to leak reports, so call them first. Once gas is ruled out, a smell that survives refilled traps means a pipe problem. The usual causes are a missed dry trap, a broken vent, or a cracked sewer line.
Nuflow DFW is a licensed trenchless sewer contractor serving 25 Dallas-Fort Worth cities, RMP# 46694. Once the gas company rules out a leak, a camera inspection finds the crack or failed joint letting sewer gas in. You keep the footage. Call (469) 701-0597.