if the code p0420 come one (cat efficiency below threshold) comes on, does that bring me into limp mode?

 

No. You'll know if you're in limp mode, vtec won't engage and you'll feel a power loss. You just need to replace your cat... This code will also show up if you're running a gutted cat or test pipe. Doesn't afftect the vehicles perfomance, and if it does, not enough to tell.