Your coolant level sensor is probably bad.

Over time, the pins to the electrical connection get corroded thus setting off the light.

For now, cleaning the pins with electrical contact cleaner will prevent the light from coming back on erroneously.

The sensor is located at the lower area of the expansion tank.

This has worked for me, although I have a new sensor waiting to be installed. FastLane has the part for about $25.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car