[screamingchief]

Too much fuel will ruin any cat in short order,they'll go nuclear I kid you not.

They'll get so F***ing hot they glow red and they'll catch the carpet inside the car on fire sometimes.

Ask me how I know about that.

Coolant is another big killer of cats,just about any car/truck that has had the headgaskets go bad,and as such has dumped a bunch of coolant into the exhaust system,will need new cats @ least 75% of the time,based on my years as a ford line tech.

Anyhow,yeah the quick & dirty test is to unhook the exhaust ahead of the cat and see if it runs better,sure it will be loud,but if it's really plugged up you'll know pretty quickly,if it's only slightly plugged then that test might not be enough to tell you one way or the other,then you gotta try something like the test pipes mentioned by the others.

Good luck with it.

Bret P.