Natural Gas Smells
First, make sure it is not natural gas you smell. If you think it is natural gas, leave your house immediately and report the odor to Consumers Energy from a neighbor's phone. Do not turn any lights on or off or do anything that might cause a spark.
Sewer Gas Odors
Sewer gas odors from the sewer main can only get into your house through your house plumbing. Every drain in your house should have a gas trap under it. That's the "U" shaped plumbing under your sinks. The water trapped in the gas trap keeps the sewer gasses from entering your house. Check to see if all drains have a gas trap. Don't forget the clothes washer drain. The drains in the basement floor should also have gas traps. The most common cause of sewer order in homes is when the water evaporates out of an unused or seldom used floor drain. The drain may be covered by a carpet installed in the basement. To correct the problem, just pour water down the drain every so often to keep the gas trap filled.
Plugged Gas Vent Pipe
Another cause of sewer odors is a plugged or restricted sewer gas vent pipe in your house. Any build up of sewer gas pressure in the sewer mains should be vented thorough the vent pipe out into the atmosphere. If the vent pipe is plugged (or does not exist), the sewer gas may have enough pressure to bubble through the water in the gas traps.
The City of Portland is committed to helping you solve your sewer problem regardless of who is responsible for the problem.