Volvo Repair: oxygen sensor?, volvo 740 gl, click n clack

Question
I was just listening to the Car Guys (Click 'n Clack) and heard what may be the answer to my problem, though it was about another type of car.  Just wanted to check it out with you.

I have a 1990 Volvo 740 GL with probably over 300,000 miles on it (original odometer/speedometer died and was down for awhile before I replaced it).  The car runs great but the "check engine" light has been on for some time.  Doesn't seem to affect anything except the gas mileage hasn't been very good.  It used to get nearly 30 mpg, but now is down in the mid-20s.  Lately I've been getting some backfiring in the muffler and just two days ago asked my husband if that isn't caused by too-rich a mixture.  (He said yes.)  With my old '66 Volvo I would have adjusted the carbuerators, but I don't have a clue about fuel injection.  

The problem on Car Talk had to do with a sulfur smell, but in diagnosing it, they asked the caller if the check engine light was on and gas mileage poor.   When he said yes, they diagnosed the problem as a bad oxygen sensor. Do you think that could be the cause of my problem and, if so, how do I replace the oxygen sensor?

Many thanks.

(Is there a way to search the questions on this site within each model of car?)

Answer
Vicki, Those guys are nuts to begin with.  It may be ann oxygen sensor but I would have to read the codes in the car's computer.  There is no adjustments with your fuel injection.  Its all computer controlled on since yours is a 90, they have 2 fuel systems on the car so I would need to know which one you have first.  I wouldnt attempt to change the sensor yourself unless you have the special socket.  The get very rusty and can get stuck.  Take it to your Volvo guys and have it looked at.  Roger