Pentair MasterTemp 400 cycles on and off


New member
Aug 15, 2019
Ok I had this problem and the high limit switch HLS LED on

I googled the issue and watched a few YouTube videos and bought a new HLS and a new Manifold By-pass valve.

I worked on it today and the problem is the thermostat not the HLS or Manifold by-pass valve.

The HLS is a normally closed 135 deg F switch that is easy to test with a multimeter and hot water from the microwave - it opens at 135F

The thermostat is supposed to open at 125 deg F and mine was stuck closed - I can’t believe they are $40. I tested mine in a jar of near boiling water and it did not move at all.

The thermostat is easy to get at and should be the first thing to check based on my experience


TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
Good info (y) Pulling the thermostat valve and testing it is on my list of PMs for the fall.

Yeah, the cost of replacement/repair parts on pool equipment is ridiculously marked up. The flow switch/thermistor in an IntelliChlor runs $100 for OEM, about $50 for generic and, I bet if you checked with the actual supplier that makes them, they probably run about $20/unit. A local pool shop here wanted over $200 for the part plus a trip charge of $80 for the tech to replace it (it’s a 30min job if you include the time it takes to find a pipe wrench and pliers in your tool box).