Fluxgate compass wrong even after recalibration

Any fluxgate compass gurus out there? I have a B&G setup. The main compass is a Precision 9. Back in late November I noticed when coming into Batemans Bay that the compass heading did not chime with visual ref points by about 20 degrees. I'd updated the chartplotter software as we were coming in once we got within phone range, as I was sick of the nag screen, which frequently reminded me there were updates available. When I contacted B&G support, they said the problem was a bug between the updated chartplotter software and the autopilot computer software, which needed to be updated also, requiring a microSD card to load it after download on a laptop. Went through all that, and a recalibration of the compass - slow 390 degree turn - and a recalibration of the autopilot, first dockside then on the water. Problems seemed to be fixed after one more compass recalibration. Heading agreed with the Square Head lead light bearing.

Next day, coming into Eden, compass heading was off again by about twenty degrees. When heading by eye into Twofold, the compass heading thought the boat was pointing south of Boyd tower at one point.

So an intermittent error of about 20 degrees.

Currently down in Tassie, and have been dealing with local certified B&G chap. Of course the first suspicion was stray metal near the compass. So shifted every possible culprit at least half a metre away from compass. Did multiple recalibrations, but compass heading would drift off not long after. So that wasn't the problem.

Next, added an extension lead and repositioned the compass away from any possible electrical interference, re-did calibrations, but problem remained.

In the last week or so it has got much worse. To the point where I have apparently mastered the art of sailing almost backwards. On a short trip under motor, compass insisted boat was headed 150 degrees off the actual heading. Needless to say this mucks up all the heading dependent information. So wind direction display is nonsense, current is nonsense, and of course the autopilot is unusable.

Next step is to replace the compass. Currently awaiting arrival of replacement. My concern is that I don't know why the compass has developed an error. And if there is an external cause, will I have a repeat problem with new compass (they aren't cheap, and they are central for the navigation system). Two possibilities I can think of is a broken winding in one or more of the nine coils used to measure the magnetic field. Another is that the EEPROM, which I assume is how the calibration is stored, has stopped working.

If so, new compass should fix the problem. But leaves the question as to whether it was just an unlucky dud compass (it's only ~2 years since installed by PO) or something has fried its innards. I have fired up the inverter, the HF and VHF radios and used a gauss meter app on an iPhone to see if there are any stray fields coming from wiring and aerial cables that are nearest the compass but have not detected anything out of the ordinary.

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Trek said..
To determine if its actually faulty or a bug something to do with the boat installation take it off the boat and try it at home with a simple battery.

I would, but I know zero about what I'd be looking for. I've fiddled with PCs and networks in the past, but never with NMEA 2000 or a fluxgate compass, so I'd be starting from scratch.

Since I posted I have connected an older Simrad rate compass on loan, and even prior to calibration it is reading only a few degrees off actual heading, so to me that suggests my compass has developed a defect.

A.new compass is on its way.

My remaining concern is whether the defect is just the luck of the draw with this particular compass, or was caused by something external. I haven't been able to find something that would do that, but I don't have the expertise to be sure I haven't missed something. So omce the new compass is in, I'll hope that's the end of it.