F430 stuck on high beam only

With a schematic it should be quite easy to find where the problem is, a false output of an ECU, wrong control signal or a short. Light circuits aren't as simple anymore as just trace a color through the loom unfortunately.

 

Saw this years ago on a 430. Was a diode in the low beam circuit that had voltage blowing through and energizing the high beam relay. Diode pz07 in the low beam diagram. If you unplug it, the high beams should go out. If you dont have access to proper electrical diagnostics and diagrams, I highly recommend taking it to the dealer to make sure there are no other issues with the circuit. Just because I had this problem does not mean yours is the same.

 

That sounds plausible. One voltage peak from disengaging an inductance like a relay or fan motor can cause that, especially if the voltage rating of the diode is insufficient. If that is the issue, see that it is replaced by a diode with a higher voltage rating and/or with a small capacitor parallel to it, to prevent the issue from reoccurring. Sounds like something that ought to be covered under warranty imho.