failed emmission test

I failed emissions test this past weekend and the HC and the CO failed!
HC=2.9253 limit=0.8000
CO=35.6131 limit=15.0000

I have heard that my o2 sensor's may be the problem? the engine is not burning all the gas ,(i can put in a half tank of gas and a day and a 1/2 later, need more gas) could this be true or what should i check first? My last tune-up was in june.

1994 chevy caprice ls 305

Sounds like it probably is the O2 sensor... Mine caused me to get horrible gas mileage, and when I replaced it, it made all the difference. Just make sure you get an AC Delco one, its a little bit more expensive, but its worth it.