I have a 2001 Sundance Optima. I recently had to replace the flow valve and the smart heater. I also replaced the high temp limit sensor. The spa appears to work fine until it reaches the preset temperature. Once reached, the spa makes a noise that sounds like water rushing through the heater then the display reads OH and it shuts down. By watching the flow valve I could confirm that the water was indeed flowing in the reverse direction as I heard the noise. The only way I can make it stop is by turning on the jets. Oddly enough, when I disconnect the power from the heater and run the spa as before it does not make the "whooshing" noise and the water does not seem to be flowing in the reverse direction. Any help in troubleshooting this would be greatly appreciated.

I'll be damned, the thing went and apparently fixed itself. Don't know why, but everything is running perfectly now. Cheapest repair ever.