'96 Catalytic Converter Issues

My '96 yukon has a service light on with the code p0420. it has 173,000 miles on it and recently the gas milage fell to about 8mpg. the temp sensor is reading normal, and i read that the code means the catalytic converter is bad. when i asked my mechanic, he told me to buy the cat set from the dealer for about $700. i really dont want to spend that kind of money but he also told me that buying any after market cat, or cat from kragen would be a waste of money because the cat would go bad in a matter of weeks. can anyone tell me if this is true? is there anyway i can just buy the converter and not the entire set of pipes? i really wouldnt care but im spending way too much on gas now and im going to have to pass smog soon. any help would be appreciated.

If you go to a good exhaust shop and buy a name brand cat such as flow master, or magna flow, you will be fine and they will last a long time. You can buy super cheep cats and they are junk. If there is an exhaust shop near you that you trust ask them what they would recommend.

I had the same problem a few weeks ago, same code. Changed my cat with a Flowmaster. Code went away I passed my emissions and it only cost me $160 installed.