In this particular case a bad cat has nothing to do about plugging up. It has to do with the precious metals in it being used up and it no longer catalyzes the HC's. I would first check the rear O2 sensor and see if it's working properly. A search should yield you the correct way to measure the voltages and see what the readings are. That will tell you if the O2 sensor is functioning properly, if it is, then the cat may be bad. The rear O2 sensor is a simple sensor and is only used to see if the cat is working properly.

An alternative method.....

If you have a IR thermometer gun, you can check the temps at the front of the cat and at the rear of the cat after it is good and hot. If the front is hotter than the rear, then the cat is likely dead. If the rear is hotter than the front by at least 100 degrees then there is still some life left in it. A perfectly operating cat may be several hundred degrees hotter at the rear than the front.

Hope this helps.

DEWFPO
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1998 S90 083,228 and 1995 964 154,100