Toyota Repair: check engine light problem, o2 sensor, v8

Question
My dad has a 2001 V8 Tundra.  He has paid over $300 twice to replace a sensor causing the check engine light to come on.  Within a week, the light is back on.  He can't get an inspection without replacing this sensor when nothing is wrong with the truck.  If you have any extended or enhanced warranty information that you could pass on, it would be much appreciated.  Sincerely, Kahne Jarreau

Answer
I'd actually like a little more information such as what the trouble codes are and what sensor was replaced. I think there was an enhanced warranty in effect but it was for 7yrs/90,000 miles so this truck would be out of this period anyway. Let me know what the diagnostic trouble codes are and which sensor was already replaced, if it was the O2 sensor I need to know which side, driver's side or passenger side?