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Spa Overheats or goes to OH error.

OH Sensors or High Limit sensors typically fault when the water reaches 112-115 degrees.

Obvious, check to see if the spa is actually overheating. If YES, then you have determine if one of several things could be happening.

First, is the pump (not a small circulation pump) stuck ON and continuously runs. Constant running pumps will add 1-2 degrees or more in a day. If you have set the spa to 104 and a pump is stuck ON, it will gain at least a couple degrees per day. So in less than 4 days the spa's water temperature could reach 112 and that sensor shut down the spa. If you have a older mechanical style spa you might be able to replace the failed relay. If you have newer electronic board spa, you usually have to replace the board to correct.

Second, is the heater Overheating. If the heater comes ON before the circ pump or #1 pump does, the heater will likely start boiling and when the sensor catches that, shut down the spa. Usually you need to replace the board. Just touch the heater when the spa goes to OH or overheat mode. If the heater is HOT, then you will need a new board.

Third, pump is not on, heater is not hot, water is not hot, but goes to OH or High Limit error. This is a challenge. Either the OH sensor has failed and telling the board there is a failure when there is not (most common) or the OH sensor is normal and the board sees something that's not there (not common).  Try going to: Jacuzzi Sensors for help. The other possibility is the board itself.