Let them replace the tach. It's learning experience for them, too, it seems. If it fixes it, you've solved your problem. If not, they'll see it hasn't been fixed and will need to continue to solve it.
I know it might seem frustrating, but they need to continue as they are directed my whomever is making the decisions at the district level for warranty repair.
other than the this problem, how are you liking the bike?
