I have a 1991 240 Volvo sedan (with over 220,000 miles on it!) and the check engine light came on. My mechanic reset the oxygen sensor. He said it looked good. A couple of days later, the check engine light came on again, and I took it back to the same mechanic. He said I needed a new air flow sensor. Another trusty mechanic told me the only reason a check engine light comes on is because of a faulty oxygen sensor.

Any opinions out there? Also, what are the signs that I need a new air flow sensor?

Thanks very much.