Hi, my hot tub has a fairly new heater, and a newish United Spas C5 box/thermostat. It started making mystery noises during the cleaning cycle, but the jets would still work. Now it's stuck on shutting down right away since it's programmed to do the cleaning cycle next. It comes on for a few seconds, shows the FLO message, starts making a chunking noise, and then the circuit shuts it off. I've replaced the filter, drained and cleaned the tub,looked for obstructions in the pipes I could get to, it still does the same thing.

Can you help me solve this puzzle? Any ideas? Thanks so much!