A toilet smells like sewer when sewer gas gets past one of its two seals: the water in the bowl’s built-in trap, or the wax ring under the base. The usual causes are a failed wax ring, a trap that dried out in a rarely used toilet, or a blocked roof vent that pulls the trap water down.
Where the smell is strongest points to the cause. If it is worst at the floor around the base, suspect the wax ring, the most common culprit. Other clues are water pooling at the base after a flush or a toilet that rocks when you sit, since even slight movement breaks the seal. A cracked flange or loose bolts keep breaking it, and a floor raised by new tile can leave the flange too low to seal. Replacing the ring means pulling and resetting the toilet, a standard toilet repair.
If the smell comes from the bowl itself, start with the trap. A guest toilet left unused for weeks in Texas heat can lose its water seal. Flush it a few times; if the smell clears, that was the cause. If the bowl water drops on its own or gurgles when other drains run, the vent may be blocked; both are signs of a clogged plumbing vent.
Less often, the bowl is cracked below the waterline and gas escapes through the porcelain. A cracked bowl usually means a new toilet.
One honest limit: if more than one fixture smells, or the odor stays after a new wax ring and a refilled trap, the toilet is probably not the source. A cracked drain line or a problem in the main line can push gas into several rooms, which the guide to sewer smell in the house covers room by room.