Any leak can skew the O2 readings.
It appears you can access the O2 sensor values in real time. Both sensors should dither, the front sensor between .1 and.9 volt, the rear sensor should be mostly between .1 and .45 volt. The ratio, front:rear, of crossings over the .45volt threshold should be greater than 2:1. If less than 2:1 the cat is bad.