it can cause misfires, which is what you are feeling when it wont go past 4,000 rpms, the cel light should be flashing when you do this, flashing light means stop doing what your doing.
if you can get your car in the air, ramps, or jacks, hit the cat when its cold, listen for stuff rattling around inside it.if so its shot..
Originally Posted by Neil98gt
its probably the cat, if it smells like rotten eggs, like stated before.
When the car is cold jack it up and tap on the cat. If it rattles, its shot.
and no it doesnt have to be from all rpm's for it to be the cat, just when it flows enough to cause a problem
I think you have found your problem.
Replace your cat
2003 Grand Prix GTP-3.5 setup, sold it, miss it
2002 Grand Prix GT-top swap, GT1 Cam, SD Headers, 90# springs, 42.5 injectors, 3.4 pulley - sold
2012 Chevrolet Cruze LTZ-Summit White, 1.4Turbo