fail pollution testing on 1999 GMC Yukon

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1999 GMC yukon , fail pollution inspection ( service engine soon light on ) Test results all passed except MIL CMD DTC4 , Po420 catalyst sys efficiency below theshold ( Bank 1 ) Is there a indicator for why my vehicle failed?
(2) Answers
This is a common failure on Chevy trucks. Here is an article I wrote that explains how this code works.

the only thing that you can do is to replace the CAT, BUT call Chevy and See if they have extended the CAT warranty for this failure, because if is so common. Best of luck
Read the bottom section on Diagnostic Theory for shops and technicians if you want to learn the why and wherefore of how this code is triggered. This is NOT an Oxygen Sensor issue. Many shops replace the rear Oxygen Sensor and the light comes right back on. I do Emissions Diagnosis and Repair in a Calif. Gold Shield Emissions Test and Repair Station. A Gold Shield Station repairs the cars that other shops have struck out on and the State sends them to me, so I am knee deep in to this stuff.

Oxygen sensors and/or too small cat converters. I had exact problem. I have same truck.