Mastertemp400 cycling in spa mode

Oct 14, 2019
2
California
Hey experts,
When I turn my mastertemp 400 onto spa mode. It will run for about a minute then turn on a HLS error light. After maybe two minutes light will go off and it will go through the same process again.

This issue first started with an E05 error message. I came to this forum to start testing ideas. Opened up my heater and was immediately shocked by how much stuff was inside. And found a nice nest had been built inside. After getting as much debris as possible out I quickly noticed that the stack flue sensor had been chewed through.
So I replaced that, but didn’t solve the cycling issue. I tested the thermal regulator after watching videos on YouTube running it under hot water and it grew also had no corrosion. Today I replaced the high limit sensor but am still having the same issue.
What do I test next? I’m worried it might be the board - but how do I test without investing hundreds on a new one?

Also maybe related:seems the temperature down and pool on button won’t work. I can’t press the pool on button for a reading of the flue stack temperature like I have seen done on here.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,809
Northern NJ
Welcome to TFP.


The HLS is often caused by lack of sufficient water flow through the heat exchanger. This can be due to several things:

  • Thermal regulator not opening.
  • External bypass open.
  • Internal bypass broken.
  • Pump too small or on too low rpm.
  • Clogged filter.
  • Scale in heat exchanger..
The bad membrane keypad could be causing you problems. You can replace the keypad for about $75.

@ps0303 @Pool Clown @swamprat69 @1poolman1 thoughts ?
 
Last edited:
Oct 14, 2019
2
California
Okay so today I took out the thermal regulator and am currently running the heater without it in. Same issue gets to about 88 degrees and starts to cycle.

Replaced stack flue sensor
Replaced high limit sensor
Have tested the thermal regulator

should I test the internal and external bypass next? Taking apart the manifold looks like a larger project.
 

GregInClaremont

Silver Supporter
Jun 25, 2017
143
Claremont, CA
Okay so today I took out the thermal regulator and am currently running the heater without it in. Same issue gets to about 88 degrees and starts to cycle.

Replaced stack flue sensor
Replaced high limit sensor
Have tested the thermal regulator

should I test the internal and external bypass next? Taking apart the manifold looks like a larger project.
Is your pool filter clean? Could there be a reduction of water flow through your system due to the filter?