2004 Ion "coolant level" message

Coolant level message on my 2004 Ion won’t go away.
I have pulled the overflow tank and cleaned and flushed it. Checked the sensor with my multimeter for continuity, checked fine. Checked and remade wiring connections to the sensor. I thought maybe the float material was the same as would be in an hydrometer so it would only float in the correct mixture/protection level of coolant. Flushed and filled the system with 50/50 mixture of Dexcool. That theory was incorrect. The only thing I’ve seen so far that makes sense is a “software issue” that was mentioned in another discussion I read.
Anyone else heard of this?

It’s likely that switch inside your overflow tank has gone bad, and it shows continuity all the time. It ought to be an open switch when you have adequate coolant. What happens if you simply disconnect it?