Another temp gauge problem

I have a Bosch electric temp gauge. Was working great went to start car gauge
pegged to 220 (cold). I've ran all the tests
grounding sensor wire, running a jumper
even replaced the sensor. Can I assume the gauge is dead or can I just test the
gauge. Like I said it was working , haven't
done anything different. Yes I did drain the
radiator . Does the sensor need fluid
in order for the gauge to operate? You wouldn't think so as one of the test is to
ground the sensor wire . Thoughts?