I am not so sure if the vehicle would fail a diesel smog test with a tune. In California I understand that the smog tester is just looking for codes or faults. With none present the vehicle will pass. I have had a minor tune on my Silverado since 2000. It is gasoline powered so they actually look at the emissions. The truck has passed every time. I am waiting for someone to see if the same is true with the Renntech Sprinter tune. We need a volunteer. For most individuals it is easier to disable the tune for the smog check and not take the risk.You will want to flash it back to stock before going for a smog inspection otherwise you will fail. Takes 2 minutes to switch it back to stock... That's the beauty of the HHT (Hand Held Tuner).
Any smog test or dealer visit you should revert back to stock.