I have a 99 GMC S1500 2wd and had P0121 and P0125 and P0420 codes showing. I replaced all four O2 sensors, reset the codes and the P0420 is the only one which came back now. I know waht the code means but how do I test a four wire O2 sensor to determine if it is a catalytic converter going bad or something else? Any help you can give me would be greatly appreciated. Thanks in advance George.