i did a search for this cause i know it has to have been asked before. but anyhow i have a 99 cav z24 with 92,000 miles on it . i bought it used ive bdone all the maintance to it that i can think of but it still runs like it has half the power. is there a way to tell if the catalitic converter is shot?

three ways

1 vacuum gage
at idle vacuum should read about 18"
cruising at part throttle it should never drop lower then 15"

2 temperature gun
the inlet temp should be about 150-200 colder the the outlet side.
the the out let is colder it is restricted if it is the same its just plan not working

back pressure gage
goes in front o2 hole
idle pressure should be less then 2-3psi
2000 rpm should be less then 7 psi
but don't quote me on the back pressure numbers.

I recommend the vacuum gage personally.
some times the temp test can lie because of thickness variation of the exhaust flanges