[91Caprice9c1]

Do you have a scan tool of your own to compare the ho2s11 and ho2s12 voltages?

How did you determine the catalyst monitor was 'dead'?

I'm of the mind that your cat is shot, if you can observe some scan tool data for me we can determine whether or not this is the case. I don't see how under the circumstances your professional mechanic reasoned that there is just some 'build up' on the cat that will go away.

If you get a scan tool hooked up drive the car around the block a few times and get it to operating temperature, then view data:

Your air/fuel sensor (o2s11) voltage should fluctuate regularly between 0.2-0.8v

Your cat monitor (o2s12) should stay pretty static and fluctuate very little and not as often as the fuel ratio sensor (o2s11). If the voltages for HO2S12 are all over the place like HO2S11, the catalytic converter is bad and must be replaced.

-MechanicMatt