I have a hot springs spa with an ace salt water system. Everything has been running fine and chemicals have been balanced withing varying ranges of normal for over a year. I went to do my weekly check yesterday and the clorine was gone. I added a little granular shock and turned the output to "boost". Re-check today, still no chlorine and the system check is flashing "0" and the system light is doing a slow blink. I turned everything off and cleaned the cell. Turned back on, added some more shock and ran the jets for 30 minutes. I'm now getting a clorine reading (probably just from the shock) and ran another system check. It is still flashing 0 with the system indicator light blinking. What should I check next? The filter looks fine too.