93 Sputtering and backfiring

My brother has a 93 cavalier that has been sputtering. I told him a while back he might try cleaning his fuel injectors and told him how to do it. Or told him to at least put some fuel injector cleaner in when he fills up again. Since he knows the guy who had it before him didn't take the best care of it, I even told him that he might try the whole fuel system cleaner and change his fuel filter.

As far as I know, he hasn't done it yet. I doubt it if he has. He called me just now and said that his car keeps sputtering and is now backfiring.

He just replaced the catalytic converter about a month ago and replaced the spark plugs in June. I did some research and I'm thinking that it might be the distributor cap. Anyone else have any ieas?

tell him to start with plug wires if they have not been done. what engine and how many miles

I have no idea on either one. I don't even know if he knows what engine he has in there. I know that motor has to have over 150k miles on it though.

change the fuel filter