Check Engine light - Honda Civic 2000

[RPT]

My 200 Honda Civic has seen 60000+ miles. Just before I had the 60k service done, the check engine light came on, and spark plugs were a tad dirty. Honda Services said there is a problem with the sensor, and hence the check engine light; they would charge me 300+ to fix (something to do with re-setting the computer in the car). I balked, and continued driving. My milegae has been low (about 60-70 miles less). Recently, the check engine light went off by itself for a full gas tank, but came back on again next time I filled gas. Mileage just that time was normal, but went down soon as the light came on. So, what may be my problem?

Thanks