There could be several reasons for the sewer smell. Here are some of the most common:
Vent Pipe Improperly Installed or Cut: If the walls around your toilet have a chronic sewage odor, it might be due to an incorrectly placed or cut vent pipe. A vent pipe can develop cracks, allowing smells to enter your house. A plumber will use a smoke machine to fill the pipe in order to identify any cracks.
Toilet Seal Broken or Loose: If your toilet smells like sewage, the seal may be damaged or loose. Your toilet may also be cracked, broken or otherwise damaged. For instance, it might have broken around the bolts that hold it to the floor; even little gaps might allow sewage gas to enter your bathroom.
Drain Blockages: A blockage in the drain line is another typical cause of a bad-smelling home. A blockage in a drain pipe will result in the accumulation of organic debris such as hair and soap. Bacteria will grow on the blockage, emitting a foul stench similar to that of sewage.