I have a 2003 LS that just started acting up. The symptoms are Under heavy throttle it stops pulling at 4000 rpm spark knocks like a mother and won't shift into the next gear until you back off the pedal at least half. It has not triggered any idiot lights/codes ? Anyone else had this issue ?

 

sounds like a plugged cat to me. Best way to tell is to remove the front 02 sensor and take it for a drive. If performance improves with the o2 out, then your cat is plugged

 

Thanks I'll give that a shot