Low coolant alarm

[870 for life]

Been looking around on the site but I cant find what I'm looking for so.
I have a 05 Volvo VNL CM870. I got a low coolant light and buzzer today. Expansion tank is full. I unplugged the tank sensor and still have the stop engine light. I then jumped the 2 wires going to the sensor and still have the stop engine light. Is the sensor just a NC switch or something different. I have been reading about a magnet but not sure if its the sensor that have. Not sure how it works yet. Haven't checked for voltage or ground yet. Just got home and its late. Anyone know how this works so I can fix my latest issue. Tank is 3 months old (aftermarket). I have schematics. Thanks for all your help