Cat replacement or 02 Sensor

[Always Wanted One]

My 4Runner started smelling like rotten eggs going up hill a few months ago - it comes and goes. Does that smell always mean a cat replacement or should I start with the sensors?

 

[Shankology]

My exp

My guess is its not the sensor. Ive never had to change my cat, but I recently had a faulty o2 sensor. The only indication I had are the VSC off and TRAC off lights turned on, as well as check engine light.