Toyota Repair: 2000 Toyota Solara Engine Light, toyota solara, diagnostic fee

Question
I purchased the Solara w/ 8,000 miles on it in 2001.  I've had it back to the Toyota dealer approximately 5 times since purchase for the check engine light being on.  While it was under warranty, they would fix it (supposedly), but never said it was the O2 sensor.  Approximately one month after the warranty was up the check engine light went on again.  They said it was the O2 sensor.  It cost me $400.  That was about a year ago.  Check engine light went on again last week.  I want to take it back but I don't what to pay for having it fixed.  What are your thoughts?  The car was a demo, not previously owned.

Answer
It may be an unrelated problem this time, I can't be of any help without knowing the diagnostic trouble code(DTC) that is stored in the computer now, it doesn't matter where you take the car they will charge an up front diagnostic fee, once you know what the code is let me know and I also need to know what repairs where done before, which O2 sensor was replaced etc.