A sewage or rotten-egg smell coming through the vents is unpleasant and understandably alarming, but it's a distinct issue from the musty or dusty smells covered elsewhere on our blog, and the actual source is sometimes not the HVAC system at all.
A dry P-trap letting sewer gas in
One of the more common actual causes of this specific smell has nothing to do with the AC system directly: a dry P-trap in a rarely used drain (a guest bathroom, a floor drain, a condensate drain with a trap) can allow sewer gas to enter the home, and once it's in the air, your HVAC system circulates it throughout the house, making it seem like the smell is coming from the vents themselves.
A dead animal in the ductwork
Rodents or other small animals occasionally find their way into ductwork or the space around an outdoor unit, and if one dies inside the system, the resulting odor can be genuinely sewage-like and gets circulated the same way. This is more common than people expect, particularly in homes with attic ductwork.
Mold or bacterial growth in the condensate system
Standing water in a condensate drain pan or line, similar to what causes a musty smell, can in some cases develop bacterial growth that produces a stronger, more sulfurous odor rather than a simple musty one, depending on what's actually growing.
What to actually do
First, check if any rarely used drains in the home have dried out, running water down them for a minute can resolve a dry P-trap issue immediately without any HVAC work needed at all. If the smell persists, it's worth having a technician inspect the ductwork and condensate system directly, since finding the actual source matters more than guessing.
Have a question about your own system?