New Member with Pool Pilot problems

We have a 16 X 32 fiberglass pool. It is 16,000 gallons and chlorinated by an Auto Pilot, Pool Pilot digital with a SC 48 cell. The pump is model E90-II and the filter is Hayward Pro Series model S244T. I replaced the tri-sensor on the Pool Pilot this past fall. The problem I am having is the Pool Pilot is showing "check flow, purifier off". I have checked everything and the flow appears to be fine. Chlorine is testing at zero, added two 40 lb. bags of salt. Could the cell in the Pool Pilot need to be replaced or could the sand in the filter need replacing? The pool has been installed for 10 years. Thanks for any advice that you can provide.