Originally Posted by N0SL3N

It is possible that the g3s are leaning out the engine which is causing faults with the O2 sensors. Did you unplug the battery when you did the intake install? Maybe when you reset the ecu it diverted to stock fuel trims.

EDIT:

This is what I found on another site

this is a *itch code. It could be an exhaust leak, intake leak, injectors, or something with your O2 or its harness.

Try the obvious:
check for cracked TP/HFC (most common)
check for exhaust leaks between cat and exhaust manifold (second most common)
check for exhaust leak between block and exhaust manifold (not very common unless you recently replaced your headers)
check for exhaust leak between cat and y pipe (will most likely trip P0420 at the same time)
check for intake leak( data log to see what your MAFs are reading)
check for faulty primary and secondary O2( easiest way is to switch the sensors and see if it trips the other side)
check for faulty MAF (data log to see what your MAFs are reading)

Crazier things to check
O2 harness (bust out the multimeter)
Injectors (have fun with this one)