the sensor could be dirty. clean off the accumulated crud with a toothbrush and if the low coolant light still remains on then the sensor will need to be replaced.
Check low coolant sensor.
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Aslo, I have found that the sensor requires the "slipperiness" of a proper coolant/water mix...too much water and the sensor will stick.
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In the radiator there is a sensor about 1/3rd of the way down from the top with 1 wire connected to it. When the sensor goes bad the low coolant light stays on. On our GM products replacing the...