the code usually appears as a result of the grey connector in front of the manifold becoming lose or the TB connector coming off
also shorting the egr wiring causes this code as well
to test the cat just unbolt the downpipe from the manifold [ 3bolts] and unplug the lambdas from the wiring harness[ ecm will use default values for 10 or so minutes]
it will be really loud but u will immediatly notice if it is your cat