No it doesn't smell like rotten eggs or anti-freeze. My son seems to think that the cat is clogged and overheating. But , wouldn't that be noticeable by a power loss? Now that I have gone out and driven the car again until it is hot, it seems to smell more like an adhesive that is hot , melting or burning. When trying to describe the smell I said dry heat... anybody who has worked in foodservice where they use warmers that have water in bottom of them- when the water evaporates and the heating unit is still on there is a definite smell and you know right away that it is a warmer that is out of water. Then the heating element is just heating up the metal in the bottom of it. It gives off a terrible odor and that is exactly what my car smells like. You cannot smell anything in the engine compartment , trunk , or outside of the car. It is only inside. I checked and it definitely is not coming from vents. What would I be looking for when checking the smart box??