1999 Chevy S-10

Your p0420 code is stating that the downstream oxygen sensor is not seeing the proper mix of gases. The problem may be the catalitic converter, but as I was told years ago "catalitic converters do not commit suicide", there is almost always an underlying cause. Unless there is a manufaturing defect catalitic converters should last the life of the vehicle.
Poor quality fuel, lack of routine maintenance (plugs, air filter, fuel filter), Oil geing into the combustion chamber (by leaking intake, worn rings etc) or an obstruction of the cold air intale (rodent nest, improper aftermarket intake system) can all cause this code to set. All of these conditions will changes to the mix of exhaust gases going passed the O2 sensor and could eventually clog the converter and/or the sensor.