19' Volvo VNL Coolant Level Critical

2019 VNL day cab with a D13, will be driving along and Coolant Level Critical warning will come on and off while driving.

I think it is either the coolant reservoir tank or sensor.
I pulled the sensor to check ohms reading.

The sensor reads 170 ohms resting, then on the top of the sensor there are two trigger points about 1/4" apart ready 2,690 ohms.

Not sure how the sensor / reservoir work together to set the warning.