Why does the coolant level warning light come on.

got a steady check engine light.  Mechanic replaced the coolant expansion
tank.  No check engine light but getting low coolant level light after driving for
about 20 minutes.  Coolant level not low.   I was told that the sensor in the new
tank could be bad or is it something with the computer??  Can't seem to get
and answer.  There appears to be no coolant leaks.