Recently, when My water pump failed, the local Pontiac Dealer replaced it and checked the codes to see why my Service Engine Soon and Service Emission System Lights were displaying. They said I had a P0139 code. Power and Ground to Bank 1 Position 2 O`2 Sensor were OK, and that I need to replace the O`2 Sensor. Now, What is the function of this sensor and what could be the result of delaying the replacement until after Christmas (about 200 more miles). I had the engine serviced just before the sensor failure and it is running good. Also, is this a difficult sensor to replace so that only the Cadillac Dealor should do it since I can`t. I know a local mechanic who is an authorized Delco Dealer and is supposed to be good with Cadillacs, and Cheaper on parts and labor. Would you let him do it and what would be a fair price?