If your catalytic converter was significantly clogged, you would notice a problem immediately. The engine would run like crap.

I would start with the cheap stuff first. Check your exhaust manifolds and your exhaust for holes. Also bad gaskets.

If the Catclean stuff doesn't work, I would try soaking each converter in dish soap and water overnight just to see if that would clean those honeycombs to allow them to do their job.

If you live in a state that has emissions testing or vehicle inspections, your car will not pass with the check engine light turned on.

 

Once again you beat me to it....OBDII (~96 up) will pick this up and set a code unless you go thru more steps to "fool" the ecu....which then brings the inspection back into question...if "they see it" they will fail you.

 

The MIL (P0430) came on about two weeks after I had my emission inspection back in September... that was close! So I won't need to worry about passing that test until Sep 2013. I'm just mainly worried about the engine being damaged by a clogged cat converter. Though I don't think my engine behaves any differently than before, my mileage has gone down a bit. But I think it could be due to either the computer adjusting the fuel system to minimize emission (thinking the cat is clogged), or the cat being actually clogged.