Chances are that when they removed the old cat's. They used an air impact gun on the bolts. IF they did not remove the the O2 sensors before using the air imact gun. The vibrations from the gun WILL damage the O2 sensors. The first thing you do is to remove the sensors. The last thing you do is install the sensors. So many people here on the forum have run into the same problem with the sensors. Nobody learns.
