I had a call about a sewer gas problem on a commercial building today. We capped off all the vents on the roof except for one. On that one we connected via dryervent hose a blower motor. We then lit a smoke bomb, laid it on the blower, turned on the blower and BINGO....we solved an issue the owner has had for 4 years. Three toilets had bad wax rings. No water leaks, just sewer gases. As gross as it sounds, I had my nose around the bases of all three toilets, but because the smell was so heavy throughout the restroom, I couldnt detect the source. Smoke that baby and I'll bet ya find the source STAT.
Hi, I have been on several jobs in the past from people with the same problem. I can tell you what I have found on a few and maybe you can look for these problems also.
You may want to look in the attic and make sure that all of your vents are going through the roof and that none of them are broken anywhere.
Like others have said already You may have a trap somewhere that is dry. Check the trap on your a/c too.
Check and make sure all toilets are sealed correctly. If a tile floor has been added and the tile guy reset the toilet, you may have a slight air gap that you do not know about. The wax ring under a toilet is also to keep out sewer oder not just a water seal.
Check to see if there are any air admittance valves in the attic or under any of your lavatories or sinks. It is usually white or black, it should be in an upright position and at the end of or top of the drain. It is used when venting through the roof is difficult of just to save some time on a job and keep the cost down. Some times they do go bad. I had many clients with that problem.
This is a few things you can check. Hope this will help you find your problem.