Something to check aswel is the vacuum lines at where you'd do the swirl/butterfly mod.
When I bought my d22 zd30 with egr blocked the engine light would come on after i dropped revs or started driving, was making me pissed big time. Changed all the filters, got a friend to look at the egr valve and cleaned all the sensors, could not find it!!
I then found that the vacuum lines were pretty bad and that and i was getting a shut down shudder which should not happen when the swirl/butterfly mod is done the right way.
On the net, bought some new samco hose and replaced all the old hose and all was good.
No more engine light!
Just something to try.