New temperature sender still alarming after engine start

[ridgy]

Yanmar 2gm, I guess either the new one or the old one is faulty somehow. Any way to test them?

Changed as routine maintenance, old one might be perfectly fine.

 

[Boater Sam]

Ohm meter (DMM) will give you the resistance. Cold it should be very high or open circuit. Warm it up to near boiling point and it should switch on.