M,
Assuming it is not the cold water shut off light... If the only light on is a single red light, then it is the cold water light. If you have any green lights, then it is not the cold water light.
As the water gets colder the tolerance between what the cell "thinks" the salt level is and the actually salt level seems to get worse and worse... The cell has an accuracy of +/- 500 ppm, so not all that accurate to begin with. I would bring the actual salt level up to 3400 or 3500 and see if that makes the cell work. If not, I would replace the flow switch assembly which is where the thermistor is located.
Thanks,
Jim R.