[CA] [CA11] How to chose the FISHFINDER temperature sensor as the data source?

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My main water temperature source from my DTS800 is showing the wrong temperature. After a year, the temperature started to increase and I calibrated it fairly often. It's now out of range of calibration, and is now showing a water-temp of more than 30C, when it's 13C in the water. The water temperature in the FISHFINDER app seems OK though. But I do not know how to make that the "System Temperature Data Source", which would solve the problem.

On my i70s Multifunction Instrument I see the following:
Transducers found:
- DST800 (which is showing the wrong temp)
- iTC-5

In the FISHFINDER app I see the CPT-S transucer.