After changing out the catalytic sensor did you erase the problem code? It is said that after you drive the car 3 consecutive times after repalcing the sensor It will see the new part has been installed and It will shut off the code. I am not so sure this is true in all cases. I have a EVAP sensor code and have replaced the selenoid just like It is suggested in the Haynes repair manual. I also know the gas cap is tight as a loose one will also give this code. I must now go under the car and check for a pin hole in the plastic tubing leading to the charcoal canister that neutralizes the gas fumes. By putting these computers on their cars Cadillac forces us to have the car repaied at their service facilities. The cost is astronomical. They even keep the software for the computer well hidden from the public that when you try to buy another computer on line. You can't find anyone who can get you one. ALL other brands are readily available. Last Cadilac I will ever buy.