99 Silverado service code p0420

[Bryan0702]

Just got this truck and the guy I got it off of said it had this code because of a hole in the exhaust. Replaced the exhaust from cat back and still have the code even after resetting the code. I've heard it is most likely the cats. Anyone ever run into this?

[anumber1]

In my experience, it is usually not a failed cat but tired, weak o2 sensors.

 

Replace both upstream an downstream sensors and go from there before the cat.

 

Personally I would do all four since the vehicle is 17 years old and has an unknown service history to you.