Engine Cooling problem
1991 Buick Regal 6 cyl Front Wheel Drive Automatic 140000 miles
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1991 Buick Regal Custom: When the car is running the low coolant light stays on. The radiator is full, and the engine is not over heating. The reservoir level does not move. Once the radiator level went from the cold level to the hot level. but won't move back down to cold even when left overnight. I don't think it's the sensor. I can only drive locally for now. What could be causing this problem. Please help!

Thank you