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the code p0420 is what the guy at O'Reilly auto parts said it was showing and i replaced all 4 oxygene sensor and the light is still on

Vehicle: 2005chevrolet silverado 1500 V8 5.3L vinB323ciFI MFI GAS OHV

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P0420 Catalyst below thresold Bank 1

What  are the code that was found? In order to accurately diagnose the problem you need to know what the ECM see with the oxygen sensor

 The ECM test and see six different ways for the 02 sensor, The purpose of the CK ENG LT is to protect the catalytic convertor, and the catalytic is to protect the enviroment. The way it does this is  to use 02 sensor, one for the upstream which monitors the engine management, and one at the rear or downstream  to reprt the condition of the catalytic convertor.The ECM test the 02 sensor for degrading which would cycle slow , it test heater circuit, it test its response time, it test if the 02 sensor is open circuit or grounded,

 With the catalytic removed, the 02 sensor can be seen, in order for the catalytic convertor to work the 02 sensor has to go from lean to rich,and this upstream 02 sensor in place in front of the catalytic will do just that, In therory the engine management was build around the 02 sensor.to help maintain a working catalytic convertor and OBD II is built for the 02 sensor.

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