In my setup (Jandy Aqualink RS), there is no sensor in the spa itself. The sensor is in the plumbing right after the pump and before the filter. It is inserted into a hole in the PVC and held with a clamp. It monitors the water circulating through the system when in spa mode (or pool mode) and turns the heater on/off accordingly.
It should not be a big deal to replace the sensor if that is the problem. In the meantime look through the manual for the heater to disable the automatic mode so you can manually turn it on to heat the spa.
Jim