Oldsmobile/Buick Repair: 1988 delta 88 repair, auto repair shop, gas mileage

Question
I've been having problems starting my car and smelling very strong odors (and getting lot lower gas mileage) after running and hearing some whirring (?) sort of noises. In the last month I replaced a battery, then an alternator. A few days after replacing the alternator the car would start but I couldn't keep it running long enough to switch from park into drive. An auto repair shop nearby put it on the diagnostic machine and said that #13 (O2 sensor was open) and #43 (ESC module circuit showed problems). We replaced the O2 chip and they reset the computer and the car is running (although idling a little rough which concerns me ). I'm still smelling some odors but not nearly as strong as before and the noise is not as noticable. Any idea what these symptoms could indicate? Did replacing the O2 sensor fix the #43 (ESC problem) or am I looking at other problems? Someone suggested I may have problems with my catalytic converter...and another suggested replacing my computer. Are those major expenses or necessary? I really need to keep this car for another year or two if possible (mileage is 118,000 one owner). I should add that my service engine light has been coming on sporadically for several years and when it first went out I had it checked at the dealership and they said it would be expensive to repair. Since and nothing was showing up at that time on the diagnostic machine they said it was okay to let it go. Car has done fine for all these years other than normal maintenance/repairs like hoses, batteries etc. My mechanic said that my transmission is probably on the tail end of its life but is not really giving me any problems  unless I'm not knowing what to look for. Help

Answer
hello jane
this sounds like a bad catlaic converter they will send off smells and the car will run rough and idile rough bring to a shop or a muffer shop and have it checked it does not sound like the tranny to me good luck to you hun and you have a pertty name pete