I have a 4000 and the
fluxgate is indeed separate.
Imho, the most common problem with this
autopilot (mine included) is bad solder connections on the
motor driver transistors resulting in drastic course changes or full lock
steering without any (or slow) return to course.
The problem arises because the factory provided no accommodation via a simple half loop in the leads where the transistors connect to the circuit board.
Heating and
cooling cracks these connections over time.
Next time mine fails (or when I get around to it), I'll add some extra length to these transistor leads and loop them to allow expansion and contraction without stressing the solder connections.