"OH" is an overheat error. In normal function, it is used as a safety shut off if the spa water temperature reaches over 110 degrees in the event of a thermostat failure. But more commonly, (if the water temp hasn't gotten too hot) it indicates that the temp sensor(s) have failed. If you have a digital display on the topside controller of your spa, then it is a circuit board controlled spa pack. Most spa builders use the same control pack manufacturer - Balboa Watergroup. The sensor kit usually consists of 2 probes and wires that plug directly onto the circuit board. Depending on the spa, they could both be located onto the heater manifold, or one on the manifold and one mounted to the tub making direct contact with the water via a thru-wall sensor mount. If I knew the spa manufacturer, I could further assist you. If you replace the sensors, and encounter the same problem... then the circuit board is bad, and usually a costly replacement. The sensors are usually listed as a "BA sensor assy" Hope this helps you,