Which model do you have?

On the DIG-220, you can enter the "test pool pilot" mode form the menu and it should cycle through salt ppm, temp, etc. and show you a voltage and amp reading (20v , 5.1 amp) for each given power level selected. I think a failing cell shows an increase in power in some cases.

You can also sample water directly in front of a return jet while the autopilot is on and test for CL.

Have you checked the cell for corrosion? Double check the cell electrical connections, etc.

How long is your pump run time?