O2 Sensor goes bad

I bought 99 HOnda Accord EX, 84K miles, one week after buying, the "check engine" light come on. I get a diagnostic done, the shop tells me the O2 sensor is bad and I need to replace the catalyst convertor ($1200). The car runs fine, does not get hot, so a friend said i probably only need to replace the sensors or I need to take the negative connector on the battery off to reset the "check engine" light. I haven't tested the battery theory yet. I live in CO, and the car was only driven in FL, the mechanic said the fuel in CO is 10% alcohol in winter and the gas just chged over, might be why the sensor went bad. I just don't know. Please advise.