Engine service light on

Maintenance/Repairs
1 
   I know when the engine service light goes on, there could be several things that is wrong with the car. I own a 2004 nissan sentra that this is happening. I understand that it could be my oxygen sensor from what the auto shop worker told me, but he has to place a instrument on it that cost $75.00 an hour to make a diagnosis. Do I really have to go through this and pay for this instrument or is there an cheaper way to get my car up to par?
2

nypdog,

Have your car’s codes scanned at Autozone or Advance for free and post the code(s) back here in number form like: PO123, etc.

Maybe somebody can give you a diagnosis.

How about engine size and number of miles on the car. Post that, too.
Is this an automatic transmission?