Follow-Up:
Your problem sounds similar to the one that I had in my Rialta at about 35k miles. A misfire on one cylinder ended up putting raw fuel into my catalytic converter, which then burned and caused several sensors before and after the cat. to go haywire for a while. I had an OBDII scanning/diagnostic device that told me what happened, and after following some procedures in the Bentley shop manual, I was able to get everything reset and working fine. Although that check engine light can come on for stupid stuff like a loose or missing gas cap, the most common reason is for misfires. Enough misfires and you will have a burned catalytic converter, so you'll want to figure out what's causing your misfires (if your cat. isn't already burned).