If the spa has a circ pump that heats the spa it could be clogged or going on u cause the spa will still heat when you are useing it cause yor spas main pump is running an allowing the spa to heat.No circ pump then it could be a few things the main off an on relay for the heater could of went bad an is stuck in the on position an when that happens the topside controls will show that the heat is off an shut down either the low speed on the main pump or the circ pump all depending on what your brand of spa uses to heat the spa. but the heater will remain on an start to boil the water in the heater until it tripps out the reset test to see if you still get power to the heater when the spa is saying that the heat is off if u have power still then its the main board an the relay. or the most coming thing can be as simple as a clogged filter in the spa an its preventing the correct water flow to go into the heater an causing it to tripp clogged filters are the main cause for most of the heating problems when it comes to the reset an everything else on the spa seems to be working ok.

What kind of appliance?