Garmin Airmar DST800 no speed reading
 

I have a Garmin Airmar DST800 triducer, (NMEA 2000 version) and the speed / knotlog function is not working.

The symptom was that over a few months it took longer and longer to start working each time I sailed. Now it's taking a few hours to come to life. It under-reads significantly, although the extent to which it does that gets less over the hours.

I have tried spinning the paddle, and it spins very freely. I added lubricant and it spins even more freely, but still doesn't work for a few hours.

Any idea what can go wrong?