2004 Chrysler Pacifica P2096

Denso o2 sensor every year I get this code this time it made it one year other times it makes it 8 to 10 months. The car runs fine like always. Last week had converter changed it was plugged. They replaced down stream o2 sensor they said it had a ding in it from pieces of the cat coming out. The upstream one is the one I have to replace every year. Went back the other day when I noticed that the muffler pipe has a slit on the top and bottom and when the shop put the cat pipe into the muffler and welded it he missed the slit in the pipe so I had him weld it shut. This is after the cat and should have nothing to do with it. I put my hand around the exhaust pipe connection and couldn't feel any leak and it is quiet. Is it just a freak thing with the ECU that when the o2 sensor gets about a year old it gets lazy and doesn't give the ECU up to date readings and the computer gives a code?