Installing aftermarket catalytic converters may trip a check catalyst below threshold code or a rich to lean code in these cars depending on the brand you bought. These motors are very sensitive. I bought a set of BBK high flows and had this issue and it took me forever to track down a simple enough remedy so I thought I might share my findings here. With the purchase of these cats I later found big daddy's garage has an o2 CEL fix. It's a cat simulator that threads between the downstream o2 sensors and your exhaust. Bought 2 at 50 bucks a pop and cleared the code without issue. Still saved me money as opposed to purchasing the higher end high flow cats as it was out of my budget at the time. These are for a racing application and not street legal in most states from what I've researched but they are great for track days getting rid of that pesky check engine light if it's a bother to you. I paired this with my borla atak cat back exhaust and a set of long tube headers and definitely make great sound and mean tone with the set up. Let me know if you have questions pertaining to hemi exhaust system set ups.

The codes were P0420 and P0430?  I've had the same codes on other cars and fix them by using spark plug non-foulers.  Drill the inside of one non-fouler to just over ½ inch so that the rear O² sensor can be screwed into it.  Now screw that non-fouler with the O² sensor into the other non-fouler.  Install all that into the exhaust bung and you're done.   O'Reilly sells the non-foulers 2 for $10; part# 42009.

Those are definitely a solid option especially with the cost difference. Ive heard mixed reviews on the non foulers specifically on the hemi platform which is why I opted for the cat simulator but it definitely could be chalked up to user error on their part for any number of reasons. Personally I had the extra 100 bucks in the budget to avoid that but if it works then that's definitely important dollars saved.

Just to explain a bit... The first 02 sensor is going to create a mild lean rich sine-wave of signal. This is normal it will go rich and then lean and back to rich and continues. Now the downstream O2 sensor is expecting a typical lean signal showing the catalysis is function by burning the excess hydrocarbons that are coming out of the engine. When the sine-wave is seen in the second O2 sensor then the P0420 code is typically thrown for a failing catalysis since its showing that same sine-wave downstream and the cat is functioning. 

 

@IBMobile posted is true I've seen a video of this showing the code issue and then using two spark plug anti-foolers it will help correct this issue.