MasterTemp 400 won't turn off


New member
Oct 24, 2020
San Jose
I have a MasterTemp400 unit that has been in place since about 2010, wherein the "Heater Off" button won't turn the unit off.
The button (and all other buttons) feel incredibly tactile and seem to be functioning.
However the Heater Off button doesnt turn off the LCD display like it does in this video @2:05

Heater doesnt have its own control box, but is hooked up to an inline timer with the Pump.
Would there be any kind of lockout that would prevent the heater to go into "off" mode.
Or is it normal for the LCD to stay lighted/on/standby while the function of the System is actually off?
Pool on/Spa on light stays on always; None of the "Service Heater" or "Service System" lights are on.

Apologies if my google/forum searches didnt already pick up on this discussion in another thread.


Gold Supporter
LifeTime Supporter
TFP Guide
Jul 3, 2013
Fletcher, OK
so does the heater actually turn off when you use the automation to turn it off?
are you asking why the led light stays on after the heater is off?

I think most units once connected to automation the onboard system stays working, when my pump is connected to RS485 it continues to work at the pump...


TFP Expert
Mar 2, 2011
Probably a bad membrane pad.

Shorting pins 1 to 2 briefly is the same as pressing "Spa On". Pressing a button on the membrane pad shorts a pin (2 through 5) to pin 1 to make a selection.

1 to 2 on.
1 to 3...pool on.
1 to 4...heater off.
1 to 5...down arrow.
1 to 6...up arrow.
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