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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.
 

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A few ideas:
1) Swap the 2 post cat O2 sensors and see if the code follows the sensor.
2) Check the wiring between the O2 sensor and the PCM to make sure the wiring is intact.
3) I remember an article online that talked about bench testing O2 sensors using a propane torch and a voltmeter. I've even done it, it isn't too hard. You just need to put a voltmeter on the sensor leads (not the heater leads) and point a lit torch on the sensor. The O2 poor environment of the flame will cause a functional O2 sensro to put out about 1V.
 

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sounds like you multiple problems at the same time you fixed the o2 sensors and if the 420 come back you have a bad cat you need to replace it. this code pretty much tells you to replace it. it is a bad cat on bank 1.
 

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Thanks to both of you, I will definetly give it a shot on swapping O2 sensors and going from there. Hopefully it will follow to the right side and then all I have is faulty wiring. Once again thank you.
 

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When you get a code for the rear oxygen sensor, it doesn't mean the oxygen sensor is bad, its notifying you that the CAT upstream is bad. But of course, if you get a code for a front, well...obviously, yes, its bad.

Also, little bit of advisement. Only use Delco sensors, sometimes denso, but don't EVER use bosch o2 sensors. They're no good.
 
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