2001 Jetta 1.8T Check engine light. wrong Diagnoses

Volkswagens are notorious for having major electrical issues. You didn't mention if the vehicle was running rough, or having issues starting. If the vehicle is runing great but the light just keeps coming on, I would put money on the fact that it is just an electrical issue that will not cause any harm. All the items that you have replaced so far would have caused the vehicle to run rough, stall out or have issues running if they were defective. The dealer should have known that and if I were you I would raise hell to them. However, if you are having engine issues then it is possible that the you will never find the problem. Volkswagen are very poorly designed electrically and the VW dealers know this. I have no confidence in any VW dealership. They will charge you just to tell you how much money it will cost you to fix the issue, and 95% of the time the issue goes unresolved. It is a Volkswagen thing. And yes to answer your question, more than likely moisture is the cause. I am currently battling to find out the issue with my 2001 Cabrio. My sister spent more than 6 trips to the Auto Shop and over $2000 until her issue was resolved, luckily it has not had a problem since. It ended up they had to replace the ground wire and Battery Cable. Good luck to you and let me know if your having engine issues, or if it is just the light.

I recently had this same problem with my 2006 Jetta. Fortunately for me I fix it be for the problem go any worse. There are many sensors though out the air Intake that can pick up on the influx of air movement to the engine and from the engine. There are many seals that connect the air intake and if a seal goes bad it allows misfire into the air intake. So I would suggest you check these seals and insure all parts of the air intake are fitting properly. I would try doing your own test. Go to the hardware store an pick up what is called pick up what is called powder smoke. They us this to find small draft in homes. In theory you should be ale to use this to check for leaks in the air intake as well. Just be easy on the powder.

Fuel Cap lose or not closed