is there any chance that the p0420 code could be caused by a loose cable? i know what it says when you look up the code online.

 

Here is what the shop manual puts forth as the possible causes for P0420, bad wiring to the rear O2 sensor could cause the problem. However the rear sensors and the catalyst monitoring system have nothing to do with engine control, all they do is check the cats and light the SES lamp if the do not seem to be working properly.

How long (months and miles) have you have the SLP catted X-pipe? It could be the passenger side cat has failed. So called hi-flow cats are just lower capacity cats, less mesh and other catalytic "stuff" inside makes them flow marginaly better then the OEM cats--but it also means they don't last as long (bet they didn't mention that when you bought it).

If most of your driving is short trips that can also shorten the life of the converters because they never get really cooking, which burns out unburned hydrocarbons that do not get fully converted due to the low operating temperatures.