I just bought a house with a hot tub. It's a Cal Spa, I think around 1990 or so. I doubt it had been used for 2-3 years. I got everything to work except the heater. When I turn the thermostat dial up on the control panel the heater clicks on, the "heater on" light illuminates, and a pump runs. However, after a few seconds to minutes the heater clicks off and the heater on light extinguishes. If I turn the thermostat down then back up the heater comes back on. However, it will continue to work for a brief period then go off again. I know the heater is actually working because I went thru this routine of turning the temp up, waiting for the heater to eventually go off then turn the thermostat down and back up to get the heater going again and after several minutes of doing this I could feel warm water coming out of the jets.

So the heater can heat, it just won't stay on long enough to actually heat the water.