I've been tuning my 04 GTO for a few weeks and got a weird error about two weeks ago. 30~90 seconds after starting the car the dash would flash "Service Vehicle Soon". The most common cause for this is the oil pressure sensor. After monitoring oil pressure that was fine as well. Car ran fine but the error would come up once each key cycle. After doing some research I read that the only way to read it was with a Tech II. There were no OBDII codes. So I started comparing tune files and noticed I disabled several MAF codes along the way:
P0101
P0102
P0103
After re-enabling these codes the warning went away. So bottom line is when tuning avoid disabling all MAF codes at the same time or the car thinks something is REALLY wrong.