Okay. I am NOT a mechanic. I have a 93 LT 1 I bought new. 17,000 actual miles. Only work ever done to the car was general service. A few years ago it began to miss and run rough once it warmed up, especially at high speeds. Didn't do this all the time and usually ran fine 55 to 65 MPH. I replaced MAP and ICM sensors. Ran a bottle of injector cleaner through it last fall and ran it pretty hard to blow out the build up. This seemed to solve the problem. It was running great until I was putting it away for winter. Then it started making a noise and wouldn't run very well. I discovered the noise was the cat converter because of where the sound was and metallic flakes and debris exiting the driver side exhaust. My question is: could this problem with the car not running well be because that the cat was failing or did the car not running properly cause the cat to fail? AND, which problem should be addressed first, replacing the CAT or investigating and repairing whatever was causing the car to run rough? I have a shop that can replace the CAT and another shop that can possibly find and repair the problem with the car running poorly. I just don't know which should be done first. I plan to have ox sensor replaced when Cat is done. No indicator lights come on. Thank you all in advance.

 

Those are very low miles for a cat to fail, so it may be a symptom and not a cause. I lost one once due to an entire bank of spark plugs not firing and all the raw fuel went kaboom in the cat. If the cat is junk, which sounds like it is, you could remove it and punch the guts out so that it flows exhaust well, then reinstall, troubleshoot and resolve whatever problem there may before putting a replacement in. Otherwise you risk the replacement.

regarding replacements, the aftermarket cats for the LT1 C4 are junk, my opinion. I had bought one and sent it straight back, they show you nice mandrel bends, you receive awful crimp bends. A used one may be your best bet, you’ll be hard pressed to find one with miles so low as your car.

Edit - reread that a shop is doing the work. They won’t punch a cat out even for testing, too much liability. I would say have the same shop do all the work, avoids the finger pointing between two shops if the end result still isn’t right.

 

Very good response. I thank you.

 

Thought I'd give you an update. Stopped by the shop today and he said exactly what you did. Gut the existing cat, get it running right and then replace the cat. He's working on a 96 right now with similar problems mine has. He suspects the injectors. Buddy I appreciate your advice.