1997 GMC Yukon Won"t start in cold.

Cold weather is here and my Yukon won't start without gas being poured into the carb. I have an engine light that stays on continous. The auto shop I went to says my cataslytic converter is going bad, but it only has to do with emission control and will not effect my performance. A backyard mechanic said it was affecting the sensors regulating fuel mixture. He said I need to replace it and the two sensors as needed, and to have the crank sensor checked. He also said that I might need to have the brain of my vehicle checked by a pro to see if it needs to be changed to accept changing fuel mixture. Are either of these guys right or should I just go to GM?