Hi,
I have a 1999 Sundance Optima 850. I recently refilled my spa. The next morning when I went to check things out, the display reads 84 F flashing with OH. An independent thermometer says the water is actually about 110F. The panel is set to 100F. Do I need a temperature sensor or a hi limit sensor? Is the temperature sensor the one in the filter compartment? Can this be replaced without pulling off the side panels? Does this just unscrew from inside the filter chamber? Do I need to drain the tub to get below the filter chamber to replace the sensor? This is the 2nd time this has happened with the spa. Is it normal for the sensors to go bad every couple of years. Is it possible that its related to anything that I am doing? Thanks!

You will likely need a temperature sensor, the one located in the filter compartment. Before you get too gung-ho, check the connection at the circuitboard for loose or corrosion. Could be that simple. If clean and tight, the sensor is out of spec and should be replaced. You would need to drain the tub to below the sensor level, since it is a hole in the side of the tub and water will rush out. You will need to gain access to the back side of the spa where the sensor is located.

You could, at least for now, just run your temp at 84-86 degrees or so, since you now know the sensor is reading around 6-10 degrees cooler than actual.