My son has a 1999 Honda Civic EX that is experiencing a rough idle, it's loud and the check engine light is blinking. We have changed the Catalytic Converter since it was rusted through, as well as the plugs. There is no visible exhaust leak. What could be causing the issue?

 

Do you still have a CEL? Rough idle could be anything. Check the coolant level. Dirty intake manifold. Clogged fuel injectors. Or the infamous dirty IACV. Just to name a few.

 

Still have a check engine light on. Would any of those things cause it to be loud as well? The noise makes me think exhaust system, but since I don't see any leaks other than the catalytic converter, which we fixed, it throws me.

 

The test for exhaust leaks is to block the tailpipe while it is idling, if there's a leak you will hear a hissing sound, if there's a big leak no pressure will build up.

Any time the CEL comes on you need to read the code(s) out and go from there.