Mitsubishi Repair: O2 Sensor Question, mitsubishi eclipse gt, most newer cars

Question
Sir,

I have at 2002 Mitsubishi Eclipse GT, 3.0L.  I recently had a battery cable replaced at the shop and when I was picking it up he told me that I would need to replace the O2 Sensor and that my Service Engine light would come back on (he must have cleared the codes).  First of all, would you recommend having the codes read at AutoZone, I think they are usually generic, do you advise anything in that arena?  

As for the O2 Sensor, I have been doing research on the Internet and see that there are two different sensors, can you elaborate?  Also, I read a response to an older Mitsubishi, is the 2002 just as easy if I buy the socket (if applicable?).  I would like to do it myself if possible.

Thank you so much for your time.

Jennifer

Answer
Jennifer,
Most newer cars have at least 2 O2 sensors.  They are known as an upstream and downstream sensors.  The first sensor measures the amont of oxygen in the exhaust before it gets to the catalytic convertor, and the second measures amount of oxygen after the catalytic converter.  The catalytic converter itself helps with emissions by burning off fuel that was not done so during combustion.  Unburnt fuel leads to poor emissions, and can indicate poor performance.  

Autozone does use a generic code reader, but it's still worth having them check it.  Since it's free, it can't hurt.  If you actually do need a new O2 sensor, you could attempt to do it yourself (AutoZone can rent you the socket for free) if you feel comfortable doing so.  This may require you getting under the car if your downstream sensor is faulty.
Good luck!