Pool and Spa
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"HL" code usually means High Limit. This is a sensor that would be in or on the heater housing. It senses water temperature and if it deems the temp to be 112 degrees or higher it shuts down the heater. Now you need to be sure the water is flowing good through the heater, if the water is not moving through the heater then it can over heat inside the heater tube. I would replace the thermostat and high limit sensor then reset tub.