If you reset the spa & it immediately reads OH & shuts down I would go after your "High Limit" sensor that is attached to your heater. also double check your filter. A dirty filter will slow the water flow passing the heating element therefore alowing the water to get hotter than it should, resulting in an overheat error.
If you reset the power to the spa & your topside starts showing a crazy temp. that the water isn't, then go after the temp. sensor. It should be in or near your filter area.

This is probally your temp sensor. Where it is on your system is hard to say. If you have a book on the system it will be in there.