There are two possible issues here. The first, as ping mentioned is a low salt alarm due to cold water temperatures. The "salinity" reading on most SWG cells are not compensated for temperature drift. Since the cell is really measuring how electrically conductive the water is (as opposed to measuring the true salinity level), the readout will drift with temperature by 100's of ppm. Cold water is less conductive than warm water so the cell will think it's low salt when, in fact, its just temperature drift.
The second issue is calcium scale. If calcium has scaled out on the plates of the cell, then that can look like "low salt" as well because, as the plates get coated, they conduct less electrical current. This is often the trap that pool owners fall into in that they see a low salt alarm and start adding salt without first diagnosing the problem. Then, when the cell is cleaned, their pool water is over-salted.
The correct thing to do here is to disconnect the cell and visually check to see if the plates are scaled. I know you mentioned that a pool service company said everything was ok, but I would not trust them. They likely went to the equipment pad, turned on the pump, checked the filter pressure and the operating lights on the SWG and called it a day. They rarely go the extra mile to really check the condition of the pool. So do yourself a favor and pull the cell. Inspect it and report back to us.
Good luck,
Matt
