i had a custom exhaust done on my car with 2.25 inch piping with a custom 2.25 inch down pipe and a high flow cat. so my check engine light came on and i went to get it check out and the cat was bad so they just put a stock cat back on. what kind of difference will going from a high flow to stock cat make

What do you mean by the cat was bad? Was it new or used, a high flow cat will alot of times throw a code. That doesnt mean the cat is bad, its a high flow which means more of the gasses are passing through it and the o2 will throw a code. My cat came with the o2 bung, I had them put a plug in the bung and my 2nd o2 sensor is up out of the way (out of the exhaust). Have not had a code thrown since.

catco universal high flow cats dont throw any codes id suggest one of them

thats what i used before on my 2200 and what i will use on my z24