I know of a trick to checking for bad cats.. two actually..
1) connect vacuum gauge to the vacuum test port on the passenger side of intake plenum between the 4 runners.. start car, check vacuum.. should be like 20-30's (help guys what's normal idle vacuum?) i think. then, rev up to 2 grand rpm and monitor your vacuum while maintaining that speed.. if vacuum steadily drops after you've maintained 2,000 rpm.. then the exhaust (cat) is clogged causing your vacuum to decrease as the exhaust pressure builds.. you will loose vacuum to raise the rpm even normally because vacuum is what keeps the rpm's down (ie: choke) but there still should be some vacuum.. if 2 grand hold a stead vacuum, try 3 grand, then try that 4 grand you have trouble with. Before doing this, i'd look on google for the process to this trick.. I but there's more defined numbers than what i'm telling you, but i had that on one of my ase tests and I don't remember the exacts.. the other thing you can try is take your exhaust manifold 02 sensor out, unplug it, set it aside and put a ball or something over the hole to sorta plug it and see if it builds up alot of pressure while running when you plug the open 02 sensor bung hole (no pun intended, lol) if it blows out alot of air just after letting the blockage off the hole you probaby have a clogged cat.. either the 1st or 2nd as mentioned.. you could also remove the 02 sensor that is downstream of the 1st cat and try the same thing and see if it's also building pressure.. if so, then it's probably your 2nd cat.. but i'd put money on the 1st cat.. You can do like I did to be sure, Gut your 2nd cat! there's no sensors after that cat so the computer will no know the difference, but you might gain a slight or unnoticed increase in hp and mpg. Been there done that. I also gutted by 1st cat years ago and that instantly sets the check engine code, but also doesn't change the engine's performance. drove on it like that for months till I could afford that new (1st) Cat.. Ebay for almost $400! and it was definitely not OE as claimed. because the oe has welded bolts sticking out of the cat to slide into the exhaust manifold, the new one did not.. I had to go by nuts and bolts to accomodate AND then they kept breaking till i got better bolts. Whatta pain. make sure it's REALLY O.E. The guy I got my cat from was from NY and that's all he sells is cats.. just so ya know. <img src="/forums/images/graemlins/wink.gif" alt="" /> . PS.. did you gap the plugs properly? what's your gap set to? Either it was coincidence the problem occured just after yu worked on it and it could be the cats or it was directly related to the work you did to it which means it should'nt be the cats unless you clogged them up by somehow dumping unburnt fuel down the exhaust (misfire).. that was my reason of clogged cats.. had a bad coil signal ground.. ran on 2 cylinders for 10 miles. Love my Kia <img src="/forums/images/graemlins/cheers.gif" alt="" />