Trouble code po420 in my 2000 sierra - 2000 GMC Sierra

The O2 sensor after the Catalytic converter has gone bad or the converter it self. Lets start with the simple and replace the cheaper of the two but before you do that, check the wire harness where the O2 sensor clips on to the wire harness at the frame. Look for any damage to the clip such as corrosion or broken wire. Good luck and hope this helps.