[arathol]

O2 sensor malfunctioning
MAF sensor dirty or malfunctioning
check for vacuum and exhaust leaks
PCV system malfunctioning / valve stuck open
check fuel pressure, weak fuel pump / clogged filter

If there is nothing immediately apparent you might want to have it checked using a professional type diagnostic machine. Its easier than throwing parts at it.

 

[arathol]

The lean code will usually reflect shortage of fuel vs airflow. because of
less flow through injectors, fuel pressure regulator, fuel pump, clogged fuel filter
or
Air is getting in after the MAF, like you mentioned -vacuum leaks from hoses valve cover or disconnected hose on the PCV system.
MAF should be able to regulate the extra air from the injen

Least expensive solution a fuel system cleaner .

A lean code after installing a CAI generally means either its not installed correctly and is leaking somewhere downstream from the MAF or the intake tubing is not sized correctly.
If its sucking in air downstream from the MAF, that air is not being read by the sensor. The sensor will only provide information on the air that flows past it, and the ECU will deliver fuel accordingly. When the leak is downstream from the MAF that will register as lean because there is only enough fuel for the air that is read by the MAF, and anything that enters the intake downstream from the MAF will lean the mixture out.
If the tubing is sized wrong the intake velocity will be wrong, so the readings from the MAF will be wrong and fuel delivery will be also wrong accordingly.

 

[arathol]

Just for purposes of diagnosis, put the OEM box back on and see what happens. If the problem goes away you'll know the the CAI was the source. If the problem persists, you'll need to do more looking.