Your 1998 VW GTI check engine light is on your running a large intake would replacing the air mass sencer turn that off or would going back to stock be the best option the intake is a eurotuned?

the only way to fiqure that out is to go and get you car scanned, then it will tell you wats wrong

Ensure that the filter is not one that needs oil. Oil will mess up you AMS. However, most of the time the engine light will not come on with an AMS fault. You will normally sense the engine is underperforming way before.