I had the bank 1 sensor 2 O2 sensor on my 01 Elantra changed by my dealer last December and ever since I have had a problem with the check engine light coming on and going off with a P0139 code. It seems that after a while it will go away on it's own only to come back on later. I brought the car in to the dealer in September when the CEL came on again for an evap leak that turned out to be my purge valve and the same P0139 code. The dealer replaced the purge valve and "cleared all codes". The cel came back on a about a month or so ago with a P0139 code and then went away a day or so after. It is now back AGAIN! I guess I'll be brining the car back to the dealer soon. What are the odds this code would come back on for something other than a defective 02 sensor that they replaced? Is there anything else that I could easily check prior to bringing the car into Hyundai? I strongly suspect they will try to deny it is a defective part/install and try to charge me for looking at the car or another repair.