Spa issues

I’m troubleshooting a Sundance spa. I believe it has an 800 series control panel on it. I neglected to get the model number. What happens is: immediately after you turn on the main power the screen comes on for a second, as the pumps fire up the screen shows OH—– and then the pumps shut off and the screen shows —-. There is no way to clear this screen after that other than cycling the breaker and repeating the procedure. If you disconnect the sensor plug the screen stays on but obviously nothing comes on either. We disconnected Pump 1, Pump 2, the ozonator, blower, circ pump and heater and tried running the machine with each one disconnected. The problem persisted. We get about 25k ohms over the temp. sensor in the heater and it drops when you hold it in your hand. Over the other temp. sensor we got about 61k ohms which I believe is sensing water temp in the spa. We removed the flow switch and manual activated it meanwhile checking it with an ohm meter. These sensors all appeared to be functioning normally. The tub was recently filled and did run for maybe half an hour after and then quit. I would really appreciate some pointers on where to go from here.