Can someone please help me out! My Furnace/central air is located in my garage, next to that is my stack pipe where my condensation line is draining into. Recently I'm getting a strong odor from that drain, now my furnace is blowing all that sewer smell(I think) through my house. Could my condensation line since it has no trap be a contributor to that or since I'm not running my A/C and using my heat has nothing to do with it?

Yep, there must be a small P trap in the AC condensate line.