I just bought an 02 silverado 1500 4.3, 102K miles, runs fine, engine light came on , P 0420 , local parts store has two , one is $470, the other is $120, do those prices sound in line, is there one out there that will fix the problem WITHOUT causing my second divorce, Wife is not impressed with having to dump $500 into a truck that I've owned for less than one month, Please help!

 

I promise you the $120 on will have the same code back in a few days to weeks. Even the $470 may cause issue depending on brand. At work we stopped using aftermarket cats due to a high failure rate during the first year. I have seen a product out there called cat a clean I would be very curious to see if it works. Not sure where it can be purchased though

 

Your problem might not be with the cat itself. After a long drive, quick get out and look to see if the cat is glowing red.
Some possible causes of your code are: clogged/broken up cat, bad downstream oxy sensor or wiring, a leak in the exhaust.

 

PO420 and PO430 are always a bad cat

 

In the causes & solutions list, I'm not seeing where his 420 code says it's a cut & dried bad cat issue: P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)
Not saying it's not the cat, but I don't know that I'd run off first thing and buy a new one without first checking some of the other possibilities.
At the very least, drop the pipe at the cat and take a peek inside before dropping money on a cat. I've had them both broken up and melted down inside, which is a big tip-off!

 

My 10 years of doing drivability/ electrical problems is saying it is a bad cat although he can waste time and money on other stuff. I do this for a living not a hobby. The modern day obdII cars are not like cars of the past. The computer determines the cat is not cleaning the fuel mixture properly when the voltage on the upstream and downstream mirror each other. The only time po420 and po430 is not the cat there would be other codes present.