I recently bought my first spa, a used 1999 CalSpa Genesis Premier. It is up and running now and seemed to be working fine at first until it wouldn't stop heating. The standard cycle turns on the jets for 3hrs every 12hrs to filter the water. During this cycle it always overshoots the setpoint. For example if the temp setpoint is 98 it will climb to 102, 105, 108, until it reaches an OH fault at 112. The actual temperature is displayed as being higher than the setpoint. The heater light does not come on while it is heating up. I have also tried EC economy mode and it still overshoots the setpoint.

When it is not in the mandatory 3hr runtime every 12hrs, I can turn on the jets and it will not turn on the heater if the temp is over the setpoint. In this case it seems to work properly. For the last week while I try to figure this thing out I've just been turning it off at the GFCI so it won't overheat. This is obviously not a long term solution and winter is coming in a few months in MN so I would like to get it working properly.

Some specs on the system (all original as far as I know)
1999 CalSpa Premier Genesis
Balboa C2000 control panel PN 27101 Rev.3
RMF Heater model C2400-0165

Any suggestions from those experienced with Cal Spas?

Thanks

Your spa is probably overheating due to the lack of ventilation under the skirt. If it's quite hot outside a spa can indeed accumulate an over-temperature condition easily due to the running temperature of the pump motor... all by itself.

As a test what I would do is open the cabinet and let it run and see if it continues to overheat. (Of course ensuring that no children or anything/one will be in the vicinity to stick their fingers where they shouldn't!)