I've read some posts about VAP acknowledging a CEL, some code apparently gets thrown about 'OEM modification' after returning to stock. Is that true? If so, can you clear the code or will it keep happening?

 

After you tune the car it will turn on the check engine light but it goes off after five engine starts (at least mine does). I think the code you are worried about is the one that gets set in the ECU but does not turn on the CEL. I don't remember what the code it but most common scan tools can't see it (or erase it) but the ones used by a dealer can at least see it I think. There are several threads on this subject if you do a search.

 

The modification code is P167F "non-oem calibration detected".
Both my iCarsoft code readers detect it, both *think* they can clear it, but it cannot be cleared.