1996 z-3 cat issue

My CEL came on back in April with a Diagnostic Trouble Code of P0420 (catalyst efficiency below threshold (bank 1). Had the CAT replaced and then within a few hundred miles, the CEL came back on with the same trouble code. Mechanic checked the car over and found that the O2 sensors looked good and that there was no exhaust leak. He chalked it up to it being a bad CAT and installed a second new CAT. A few hundred miles later, same thing happened. CEL light came on with a trouble code P0420. I am going to go back to the mechanic but what could be causing the CEL to keep coming on with the same trouble code?

i'm thinking an o2 sensor issue, just a swag though..

GG