You stated initially that the pool store tested your salt level at 3000 ppm and you have added more salt. Forget about the reading on the control panel. That is measuring conductivity inside the cell and if something is bad it won't be accurate. The unit will shut the cell off when the salt level gets much above 4000 ppm. Get your water tested and please post a complete set of numbers. If the check cell light is coming on then your salt levels might very well be too high even though the display is reading low! Yes, you probably do need a new cell from what you have said or you have not gotten all the scale off of your cell and further cleaning might help. How does the cell look on visual inspection. Is there any visible scale? If you can see it clean it again.

Hope this helps.