Pentair Mastertemp 400 displays 126, already replaced thermistor

PowderDay

Member
Dec 16, 2018
9
Jacksonville
I have a pentair easy touch system with a pentair mastertemp 400 heater. The heater won't turn on because it's displaying 126 as the temperature. I replaced the thermistor twice so it's not that. I also tried replacing the control board on the mastertemp 400 and it was still displaying 126. Any recommendations on what to try next?

Thank you!
 

JamesW

TFP Expert
Mar 2, 2011
16,080
Are any red diagnostic LED’s on back of board (AGS, AFS, SFS, HLS, PS or THERMISTOR) “on”?

Is red “SERVICE HEATER” LED “on”?

Can you describe what happens when you first power on the heater?
 

PowderDay

Member
Dec 16, 2018
9
Jacksonville
Are any red diagnostic LED’s on back of board (AGS, AFS, SFS, HLS, PS or THERMISTOR) “on”?

Is red “SERVICE HEATER” LED “on”?

Can you describe what happens when you first power on the heater?
When i turn the spa on from the easy touch panel, the auto valve moves to spa mode. Nothing happens with the heater though. It displays r13 then 126 degrees. The only LED light that comes on is LED10 (which is the "spa on" light)

I can't find the thermistor led light but no other lights are on. Ther red service heater light is not on.
 

JamesW

TFP Expert
Mar 2, 2011
16,080
If you have replaced the thermistor and control board, that leaves the wiring as the next component to replace.

Maybe mice got in the heater and chewed the wires?

I think that the following are the LEDs:

1.....PS (Pressure Switch)
2.....HLS (High Limit Switch)
3.....SFS (Stack Flue Sensor)
4.....AFS (Air Flow Switch)
5.....AGS (Automatic Gas Shutoff).
6.....Service System
7.....(Thermistor?).
8.....Heating
9.....Pool On
10...Spa On
11...Service Heater.

I think that led #7 was supposed to be thermistor, but it doesn't seem to be on any boards.
 

PowderDay

Member
Dec 16, 2018
9
Jacksonville
If you have replaced the thermistor and control board, that leaves the wiring as the next component to replace.

Maybe mice got in the heater and chewed the wires?

I think that the following are the LEDs:

1.....PS (Pressure Switch)
2.....HLS (High Limit Switch)
3.....SFS (Stack Flue Sensor)
4.....AFS (Air Flow Switch)
5.....AGS (Automatic Gas Shutoff).
6.....Service System
7.....(Thermistor?).
8.....Heating
9.....Pool On
10...Spa On
11...Service Heater.

I think that led #7 was supposed to be thermistor, but it doesn't seem to be on any boards.
I ordered a heater control board from amazon and installed it but it still shows 126. The board i ordered was "Pentair 42002-0007S Control Board Kit Replacement NA and LP Series Pool/Spa Heater Electrical Systems."

When I received the board i noticed it said STA-RITE and AFTRMKT on it. The board hooked up fine. Still displayed 126 although it was really hard to see because the display is different than my existing board. Anyone know why that is? My existing board had big red numbers and the new board has faint black numbers.

I don't see any signs of mice or chewed wires. Heater is about 10 years old. Any chance it might be the easy touch control board? Although everything else on that seems to be working fine.
 

JamesW

TFP Expert
Mar 2, 2011
16,080
The Pentair MasterTemp and the StaRite Max-e-Therm are the same heater in a different case.

They use all of the same parts. If it's a StaRite part, it should be ok.

However, it sounds like you got a non OEM part that might or might not be ok.

What is the part number of the new board?

I would try new wires.
 

PowderDay

Member
Dec 16, 2018
9
Jacksonville
What do you mean by new wires? There is a ton of wires in the mastertemp 400 heater.

I was hoping to fix this myself but it might be beyond my capabilities. I have my pool store tech coming out Monday but they said they can only fix it if it's the automation. Then i have a gas company repair tech coming Thursday but they can only fix the heater. I still haven't isolated which is the problem.
 

JamesW

TFP Expert
Mar 2, 2011
16,080
The wires that connect to the thermistor. Maybe disconnect them, tie a string to the ends and pull them out to make sure that they are ok.

If they are ok, pull them back through with the string.
 

PowderDay

Member
Dec 16, 2018
9
Jacksonville
The wires that connect to the thermistor. Maybe disconnect them, tie a string to the ends and pull them out to make sure that they are ok.

If they are ok, pull them back through with the string.
Is there somewhere i can find a diagram of which wires are for the thermistor? I already replace the thermistor and wire that goes to the easy touch control board. I'm assuming there is a wire that goes from the easy touch control board to the heater control board?
 

JamesW

TFP Expert
Mar 2, 2011
16,080
There are two wires that connect to the thermistor and they go to the control board.
 

ps0303

TFP Expert
In The Industry
Jul 6, 2011
4,006
FL
The 126 is the normal code displayed at boot up of the heater. What temp is the ET set to? What temp does the ET say the water is? Is the heater also cranked all the way up to 104 on the spa setting?
 

JamesW

TFP Expert
Mar 2, 2011
16,080
Normal boot up is 888, 128 and then r8 or r13.

The numbers are displayed briefly and then go to water temperature.

I think that 126 is the highest temperature indicting a thermistor error.

OP, can you describe the startup display?

Can you measure the resistance of the thermistor?

What was the part number of the new thermistor?
 

PowderDay

Member
Dec 16, 2018
9
Jacksonville
Normal boot up is 888, 128 and then r8 or r13.

The numbers are displayed briefly and then go to water temperature.

I think that 126 is the highest temperature indicting a thermistor error.

OP, can you describe the startup display?

Can you measure the resistance of the thermistor?

What was the part number of the new thermistor?
When I turn the easy touch control to spa i see the heater then display 888, 128, r13 all very quickly. Then 126 comes up and stays up forever. When i look back at the easy touch display it shows 60 degrees / 101 degrees. 60 is the water temp and 101 is what it would try to get to.

The first thermistor i installed was from amazon. Part number 520272. That didn't fix the problem so i then bought a thermistor from my local pool store. That one was the exact one my system had before it was broken. Hayward Part GLX-PC-12-KIT. This didn't fix the problem either.

I don't know how to measure the resistance.