What are the symptoms of a bad catalytic

Usually a bad unit will heat up drastically. Making it turn bright red under the high temperatures. It will cause the vehicle to run rough and even backfire. The unit is located, usually, under the cab along the exhaust piping. It looks like a muffler but it's too far forward for that, shaped slightly six sided and should be welded in place. It's not hard to replace if you have welding experience, however it is an expensive item. You should be very sure that is the problem before replacing it as they are usually designed to last a very long time and not fail. Good luck to you