01 civic. 160K miles. Still going. Yesterday check engine light came on. Manual says emmissions problem and should be fixed. My mechanic says it is a "code P0420" and it is an oxygen sensor. Claims there are three sensors and its about $65-100 PER sensor. He cannot tell which sensor is bad??
His suggestion: Ignore light. "It will not hurt anything. Just a small drop in MPG"
Any suggestions? Find a new mechanic? I only have one dealer in my area and I have had 2 bad experiences so would like to avoid if possible.

Thanks