Stalk fail is an error over the seatalk line that you can only see over the display of the autopilot (no other instrument complain about it).
90% of the cases the problem lies in the autopilot controll unit (smart pilot s1, s2 s3), in the other 10% the problem is a discontinued or damaged YELLOW cable from the seatlk line.
When you disconect the seatlk lines from the autopilot with a meter between braid (grey) and yello colors of the seatalk you should have like 10V pulsating- if you don't the problem lies in the autopilot controll unit. One easy test is also to disconect all the seatlak lines from the autopilot and only connect the autopilot display with 3 wires: red, yellow and grey...
Another problem sometimes is inside a multifuncion display that's connected on seatlk line, there is a secret Raymarine internal CONFIDENTIAL document that show how a certain component fail and that cause the problem, forr testing purposes the same thing, disconect the seatlk from the MFD and measure with a meter between the yellow and braid- DO NOT USE THE NEGATIVE WIRE OF THE BOAT but the braid!
that was an S3 with seatlk fail... one of the first videos I've made sorry for the bad quality
how to diagnose the fluxgate
problems on raymarine (all the fluxgate from autohelm
to the last spx are the same) even if is in italian you can understaind wha'ts going on and what the voltages are...