I have had problems with my GFCI tripping on 1997 Leisure Bay 19100L series. Usually after I wait a few days I have been able to reset it. I recently replaced the GFCI and have the same problem occuring. When it is not tripped the circulation pump is working and has normal flow. Also the heater is circulating hot water. Then if I check it the next day the water is around 100 degrees but the GFCI is tripped and wont reset. The digital readout that displays the temperature on top side is jumping from 138 to 155 even when I first start it up and numbers going up from there. The switch to change the heat is a dial and I can hear the heater kick in as normal when I turn that up. I was wondering if it could be the high limit sensor or the themostat causing my problem. It appears to be heat related but even when I cool the hot tub way down the GFCI will not reset unless I wait a few days? I was about to order and replace the thermostat next but wanted an expert opinion first.

Replace the heater element and the topside control panel.

also- try unplugging the ozone generator, if equipped.

The digital readout in your spa is essentially just a thermometer. It's not hooked up to the thermostat or heating system in any shape, way, or form.