shi,
Yes.. the IC40 uses a temperature sensor to help determine the salt reading. So it may well be the temperature that is causing the low reading. The problem is that the IC40's salt sensor has a range of +/- 500 ppm.
When your PB tested the salt level, what was the reading.. "fine" is not a very accurate test.. If the reading was near the low end, I'd another bag or two of salt and see what happens.
That said, It is getting to that time of year where the cell will shut off, I think about 52 degrees or so. It might make sense to just chlorinate with bleach until time to close and see where you are next spring.
Jim R.