2005 Chrysler PT Cruiser O2 Sensors

Hi my name is kyle and I have an 05 pt cruiser stock and I have had problems with the o2 sensors since the first few months. I have pulled codes p0138 p0139 and I think p2097 over the past year and a half. I first brought it to the dealer and they replaced one of the sensors. The problem stopped for about 6 months then the same codes appear. I brought it back to the dealer and they said the o2 sensors were firing fine. I have spent alot of money only to hit another dead end. What could be prompting these codes? What am I doing wrong? Please please help