2008 gmc sierra truck will not pass emissions

Question

replaced catalitic converters

Answer 1

Will not pass emission test, is the monitor ready? Any pending code that is stored, These will prevent the vehicle from passing its emission test

 In order for the catalytic convertor to work properly it need to go from lean to rich, by doing so the catalytic can do its job in remove harmful emission emitted from the engine

 What make the catalytic convertor to work properly is a upstream 02 sensor, the downstream 02 sensor monitor the catalytic efficiency,

 A good working upstream 02 sensor will put the engine  in fuel control. Here it goes from lean to rich and at a proper hertz. This engine is in fuel control.