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Discussion Starter #1
My SES light came on (not blinking) on my '99 V8 with 67K miles. I ran the diagnostic and got a P0420 code -catalyst efficiency below threshold. I'm wondering if anybody has solved this problem recently and got the SES light to go off permanently. So far I've done the following:

1) Replaced the cat back on the front passenger side (which my mecanic swore was the problem).
2) Replaced all spark plugs.

Replacing the cat and resetting the computer worked for about two weeks but the light is back on. Next step would be replacing the 02 sensor but I'm hoping to hear what's worked for others before sinking more money into this. I have a Vortech charger which a dyno guy said puts added pressure on the cats (possibly but doubtful, dude). I know this is a somewhat common problem so any help would be appreciated.
 

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The Catalist threshold is your Catalitic converter... not your cats back or your plugs.

Your mechanic should have known that. Your catalitic converter is either a) clogged all to hell, or b) going/has gone bad.
 

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Discussion Starter #3 (Edited)
Thanks for the reply. Ok, my bad here. He did replace the bank 1 catalytic converter and now the SES light is back on after a few weeks. Could it be something else such as the O2 sensor?
 

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Just to close the loop on this super popular thread, the new cat that I replaced two weeks ago was rattling like crazy today so obviously it is defective. I think the cat on the passenger side is rattling as well so I will likely replace both of them - one I will have to pay for and the other one should be free since it died after two weeks. The original cause of all the problems - likely 60,000-plus miles with a Vortech blowing up a cats ass.

Just to share what I've learned about the annoying P0420 code:

1 - The catalytic converter may be clogged all to hell or has gone bad (like Evin said). Replace the cat(s).

2 - Your O2 sensors may be fried and your engine is overcompensating what it thinks is a lean fuel mixture. Replace the O2 sensor (much cheaper than replacing the cats so hope that this is the problem).

3 - A supercharger will hose your cats after 60,000 miles without a doubt. Mangaflow, Flowmaster, etc. They will all be dead eventually.

I've probably read every Website post possible on this problem so here it is in a nutshell. Hope it helps somebody eventually.
 
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