[TG] ST70 and DST800 speed calibration issue

[Azura]

My boat is equipped with full Raymarine instruments including ST70 displays and DST800 transducer. Recently the boatspeed reading has been indicating faster than actual speed by over 1 knot at full speed, although it seems very close to gps speed at lower speeds. I have gone through the calibration process multiple times, and the more data points I add, the less accurate it becomes. I even tried a factory reset and went through the whole commissioning process from scratch.

My most recent attempt at calibration attempt resulted in a boatspeed reading far below SOG, even though 1 of the calibration points I used was exactly the speed I was going.

All of my calibration was done in minimal current conditions. The impeller is clean and spins freely.

I also noted that the water temperature displays much lower than actual temp, and outside the offset range.

I have read about testing the transducer with a multimeter, but my DST800 connects directly to the STng bus, with no pod or ITC-5 module.

My questions:

Is there a way to test my transducer?
Could the problem be caused by a problem on the SeatalkNG bus, and if so how can I troubleshoot that?
Could the problem be related to the ST70 displays?

I welcome any suggestions you might have!