I just bought a house that has an old Autopilot LS1000 system with a RC-15 Cell. The red flow light was on so I pulled the tri-sensor to test it and found it to be in poor shape. the plastic around the temp sensor was cracked and corroded andthe flow sensor would not work when checked for continuity. So I replaced the tri-flow sensor and now the green lights are on for the flow, salt and temperature.
Now when I run the system, I either get a red cell light or a flashing red and green cell light. This seems to change every time I cycle the system on and off. Is this an indication of a bad cell or control board? Also, I checked the cell cord for continuity and it was ok.

if its a bad cell, what replacement cell should I consider? I estimate the pool to be about 18,000 gallons.

I have a feeling the system hasn't been working for a while. The previous owner had a pool service that would come twice a week and I believe they were adding chemicals. I'd like to get the system working again if possible.