have the wave forms of front and back sensors looked at first. if rear sensor level is low and flat then the sensor may be the issue. if it is active and looks like the same kind of activity as the front then the cat is likely the issue. and you can just bolt on new complete cat assemblies if you like.
I'm going to take a peek underneath it first to see if the new sensor worked itself loose; hopefully that is the case and I fix it. If not, I'm taking it in Wednesday for an oil leak fix and I'll see if they can scan it then.