i've had this code a while (catalyst below threshold bank 1)... i got a pacesetter header 2 1/4 piping all the way back with a magnaflow cat and flowmaster 40 muffler. i've been running into some trouble with the law so i want to get rid of the big red racing sticker i currently have on my windshield. i was wondering what fixes people have done to cure this code. i've contemplated changing to a different cat, but have no idea which one would help. would inserting a resonater into the mix help solve this code. ? i've also recently acquired a p0480 (fault with cooling fan circuit) not exact words, but can't remember. i've heard rumors that these codes run hand in hand. i don't see that as possible... but you never know with these heavy chevy's.

it may be a fouled up sensor or a burnt out cat.... when I had my 96, I changed to a pacesetter header, 2 1/4 piping all the way back to dual magnaflow mufflers, stock cat...no codes. later I swapped out into a 2 1/4 high flow cat...still no codes, added a cherry bomb as a resonater..still no codes. I can't see how what you did would give you that error...less the sensor got damaged...or the cat got burnt out...but those things are supposed to last for a while

Found this..it might help.


3. My check engine light came on and my mechanic said the Catalytic Converter was the problem. How can I be sure?
This has been a tricky problem for most mechanics and exhaust shops, When diagnosing the problem with a scan tool. The code that comes up for an inefficient Catalytic Converter is a PO420. This is an indication that the Catalytic Converter is not doing its job properly but we have found that this code could also occur if the Oxygen Sensor (O2 Sensor) or sensors become faulty or lazy, We have also found that on the Jeep Cherokee, Subaru Outback, Ford Explorer and Honda Accord, the factory tolerances that set off the check engine light are very tight and an engine that has carbon buildup in the combustion chamber will bring up the cylinder head temperature which in turn brings up the Oxides of Nitrogen (NOX) and will set the light off. In these cases we recommend a product on the market that has been working well to cut down on the carbon build up and will reduce the dreaded check engine light and PO420 code. The product is called BG44K- Total Deposit Control Fuel Additive and can be purchased from us at our product link. A good rule of thumb here is to make sure all components that we have mentioned here have been checked before the Catalytic Converter is replaced. There is nothing worse than spending hard earned money on a Catalytic Converter and find out that the check engine light problem still exists.

take from http://www.longbeachmuffler.com/faq.html#5


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