my autoenginuity software reports "P402" as
EGR-FLOW EXCESSIVE DETECTED
im just guessing but i BET if you take that intake off
and put the STOCK intake back on the code will
go away.
i know for a FACT that the exhaust <alone> will NOT set the above
code...
I see your from california, am i right ?
Is your CEL or MIL (check engine light or malfunction indicator light)
on ?
on california trucks emissions codes (like egr) should turn the
light on.
personally id wanna CHECK/TEST the egr valve (autoenginuity does
that)...to determine if the valve is sticking (open or closed) before
i considered replacing the valve or putting the stock air box back on the
truck.
on the other hand (im thinking/working thru this as i type) you
would be getting different codes if the valve were opening and
closing erractically or sticking.
so i suspect its your highflow/aftermarket airfilter (you dont say
what brand/size exhaust or the brand/setup on the intake either)
i HIGHLY recommend you fill out your profile, year, model etc of
your truck...it would be helpful.