Mitsubishi Repair: O2 sensor, eclipse gs, exhaust pipes

Question
i have a 2001 eclipse gs, 2.4
the service engine soon light have been coming on. i went to get it checked out. i was told it was my rear o2 sensor. i changed it once. then they told me i had a bad o2 sensor so i bought a O.E.M rear sensor. the light is still on. i went back to get it checked out. now im told that i have a bad heater ciricut from the computer the the o2 sensor?  they tried to reset it again. but the light keeps coming back on. how do i fix this. i would like to do it on my own

Answer
Jerry,
If you were taking your car to a local mechanic, you may want to get a second opinion from a dealership mechanic.  If you have been taking it from the dealership, you may want to speak with the service manager and ask why they cannot figure out the problem.  The dealership should be able to figure this out, unless the computer is actually malfunctioning.  Replacing an O2 sensor is not that difficult, if you have experience with replacing exhaust pipes.  But there may be nothing you can do unless you find out exactly what the problem is.  I'd have the dealership inspect it, if you haven't already, so the actual problem can be pinpointed.
Good luck!