Catalytic converter

How do you check a cat to see if its bad?

Most if not all cars will tell you when the converter goes belly up. You should be getting a CEL (check engine light) if it is bad. Have it checked for CELs and if you have one or more, get the actual numbers (format = P1234)

Why do you think it is bad. How old is your 2500 and how many miles on it?

Depends. Checking catalyst operation may be over your head as to equipment and expertise. Checking for a partially clogged converter can be done with a vacuum gauge.
In some milder cases one may find a vehicle with no noticeable symptoms but the leading edge of the substrate in the converter will be partially clogged. The only way of determining this is by cutting the cat open, which means new cat of course.

You have high HCs so first off you need to make sure there are no hints at all of an ignition problem. This means plug wires, dist. caps, etc. and just my opinion here, getting rid of those worthless E3 spark plugs you have. Go back to what the factory recommends and properly gap them.
Make sure the ignition timing is set properly and use the test plug when doing so. If need be, add 1 or 2 degrees more advance to the timing as this has the effect of leaning the fuel/air mix out.

There’s a lot of things that could contribute to high HCs and the above is only a small part of it.

On modern vehicles, two oxygen sensors (before and after) monitor the performance of the converter…When it gets weak of fails to function, the readings from the sensors cause a CEL to come on and set a trouble code in the OBD-2 computer…The code translates into something like this: “Catalyst performance below threshold”.

Before you rush out and replace the converter, change the rear oxygen sensor as it is the part that is generating the failure code…