Volvo: Check Engine Light, oxygen sensor, 30k

Question
Hi Jason,
Can I purchase one of these computers?  I've seen a few for sale, ranging from $125-$200.  Would the readout mean anything to me?
Or,  Can I change the 02 sensor and if that was the problem, would the light go off on its own or do I have to reset it somehow?  02 sensor has never been changed, and I am around 74,000 miles.

Thanks,
Bob
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Followup To
Question -
My Check Engine Light came on for a few days and then went off on its own and has now come back on.  
Car running fine.
How can I determine what the problem might be and possibly do it myself?
Thanks,
Bob
Answer -
Unfortunetly if there are no physical symptoms, it's really hard to find the problem yourself.  Usually the only way is the take it in to the dealer and have them hook it up to the computer and look for an error code which will give them clues where to look.  Most of the time it is a sensor beginning to fail somewhere, like they oxygen sensor.  Hope that helps.

Answer
You can purchase these computers, although I'm not sure from where.  I would suggest posting that question at www.brickboard.com, there a many members there that I believe use them.  As for the oxygen sensor, you never told me what model and year Volvo you have, so it's hard to tell if you need to replace the oxygen sensor.  New models will go 100K before replacement, but in my 1980, I have to replace it every 30K.