then visit your local muffler shop and for about $50 and 5 minutes on the lift they will change it. Mine even reset the CEL. Been there, did it twice...one on each side. Same price two different shops in two different states. 5 minutes. Or egt under the car and do it yourself.
BUT the 420 code is not an O2 sensor code but a catalytic converter code. The flow chart for curing it says cure all other faults first. Any oiled air filters ever used? Checked the throttle body? AOS good? What would cause a CAT to fail?
http://sites.google.com/site/mikefocke2/oxygensensors will give you part numbers, sources, how-to, etc. if it turns out to be a faulty reading sensor.