Nearly Stock KA didn't pass smog...ideas?

[Jimmyg41]

Well it was smog time and I failed pretty miserably. I got 5.0something on the Co2 part which you're only allowed to get a 1.0 on. I reset my ecu to take off the check engine light but before it was throwing a code that said I had a bad front o2 sensor. I told the guy at the smog place and he said that was a problem but wasn't the whole problem. I also thought that maybe my cat was done for or something but does anyone else have any ideas? I have a 95' 240 and it does have some miles on it, about 136k to be exact but no real problems. I do have crappy gas mileage and a rather erratic idle. Any help would be cool...thanks.