Pool and Spa
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You either have a bad limit switch or the temperature is overheating and shutting down on high temp limit.
First thing to do is verify you have proper water flow.Are all valves open and can you verify we have at least 30 gallons per minute running through heater?
Personally if valves are all open I would contact installer rather than messing with the unit as you do not want to void warranty and if there is an issue with a defect they will fix it.
However if this is something you want to troubleshoot and you know we have good water flow, we will go from here.
Peter I have to leave for a bit but will check in when I get back. If you decide to troubleshoot. Below is procedure:
1.Inspect temperature limit switch wiring
2.Remove wire leads from limit switch and jumper leads. Operate heater. Measure continity across limit switches. If closed, LO code is not caused by temperature limit switch fault.
3.If open, Remove jumper from leads and reconnect leads to temperature limits.Verify that water flow is adequate.Verify that water flow rate to heater is above the minimum required , 30 GPM for H350FD and H400FD)
4. If OK, Temperature limit switch is defective.Replace temperature limit switch.