Pool and Spa
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That is an error message for High Limit failure. The high limit sensor is designed to read the temp of 110 degrees and will shut down the heater until resolved, now most of the time when this error comes up it is because the temp sensor had failed and the water temp has risen to 110 degrees but if your water is cooler than that and you still have High Limit error code then most likely you need to replace the high limit sensor which is located on or near the heater housing. Find on the circuit board where the sensors plug in and trace back wires to sensor then replace. In my opinion I would replace the temp sensor as well since they need to work in harmony with each other better to have brand new ones.