The answer to 'why is my sewer line foul smell' is that a foul odor coming from your sewer line is typically caused by a blockage or buildup in the line. This can allow sewer gases to escape and cause an unpleasant smell in your home.
Some common causes include grease buildup, tree root intrusion, broken or cracked pipes, and items flushed down the toilet that should not have been. Symptoms include a strong sewage-like odor, slow draining sinks or toilets, and gurgling sounds coming from your plumbing fixtures. To fix this, you may need to use a plumbing snake or have the line professionally cleaned. If the problem persists, you may need to have the sewer line replaced.