A cat is basically an oven or thermal reactor:
O2 + fuel + platinum/palladium =makes heat combustion to split the stray molecules that aren't burnt in the combustion chamber.
Test:
1). Put an infrared thermometer on the tailpipe after driving a bit.
If working- It should be 50-100deg hotter downstream IIRC.
If not- Both temps will be the same.
2). If it's clogged- Performance off the line will be good, then start to fade as back-pressure builds up in the tailpipe.
(remember that no air out equals no air in
)
I've been told cats can possibly be cleaned by super heating (baking in oven or 'italian tuneup' style), but the thin platinum/palladium surface will eventually wear off.
If silicates from RTV gasket sealer usage have coated the surfaces, there's zero chance of revival.
There are also cheap back-pressure testers before $pending money, but the code could just be a bad O2 sensor/wire.