Problem with 1989 535i E34

Although this car isn't mine, (My Dad's, want to buy it off him sometime when I'm old enough) I thought I had to ask this question.
Whenever he turns the car on or off a *bing* sound goes off and the little dial thing says "Coolant level". He's checked if there's anything wrong with it, and there isn't, so how can we fix it?

If you can turn off the check indicator and silence the bell with a shunt on the sensor, you just need to replace the sensor.

Shunt is a wire from one wire to the other, bypassing the sensor to test the rest of the circuit.

Later,