Worked on several P6042's with Hall sensor problems.
I suggest you should not wast your time (or money) trying to repair.
I even bought a new (old stock) hall sensor (kit) from Sphere but it turned out to be completely useless
(much worse than the original)
I think the sensors just deteriorate with time if you use them or not.
Unfortunately it is very expensive technology to make such sensitive (and wide bandwidth) probes.