Maxima smoking and sputtering

My 99 maxima has been losing power and smoking from the bottom of the engine for a couple of days, I was told that it could be the cat converter. It uses no oil or water and doesnt overheat. When I got home from work I noticed that there was a fire underneath my engine and this is why the smoking. I put it out, any ideas what it could be? The smoke is white and sweet smelling but the car doesnt use any water or antifreeze, also it sputters before it smokes and my brake and battery lights come on the dash. Thanks.