Just opened pool but heater won't start


New member
May 18, 2020
My Pentair Mastertemp 400 goes through its electronic startup routine and then reports a HLS error (and service heater led). My water is circulating normally, my chlorine generator is saying that it is working but the heater stops as quickly as it starts without lighting or blowing. I removed the Thermal regulator and that, when tested, operated perfectly. If I reduce the temperature setting of the heater to below the reported water temp (66), the Service Heater led goes off. Does that imply the circuit board is fine? I don't know what I am supposed to do to bypass the HLS sensor to determine if it is the problem. I will open the manifold if necessary but it is a biggish job and I don't want to do it unless it is necessary. Any clues as to what I can do to determine the most likely cause of the problem. It worked fine prior to closing down last fall and I haven't heard the heater fire up yet. Gas is on. Opened the case and didn't see any animal habitat, no chewed wires.Any help would be greatly appreciated.


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
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@ps0303 @Pool Clown


New member
May 18, 2020
What do you know. I jumped the wires on the the HLS sensor and the unit started up first time. So I now have an order for a replacement and in the interim, I will let the heater increase my pool temp by 18 degrees before I disconnect the bypass. Thanks for all the help.
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