Pentair 125 showing 126 on display; thermistor resistance seems ok

[El-Guapo]

Hello! My kid just asked me to heat up the spa at our vacation house. When I turn on the Pentair 125 heater, the display shows "126". Research online suggests that this is telling me that the system thinks the current water temp is 126 degrees, and that the prime suspect is a bad thermistor. I found a page here with a table of values of ohms vs resistance. My ohmmeter is showing me 13,500 ohms, which correlates to a temp of 65, which seems right. So it appears that the thermistor is working? I tried jiggling the wires on the When I remove the leads from the thermistor, the temperature on the display rapidly drops.

Most advice says "replace the thermistor!", which I'm happy to do, but my ohmmeter reading suggests that the thermistor is working just fine. We're only in town for a week -- if I order the thermistor and it isn't the problem, then 2 days of our vacation are already gone. I'd like to do this once and get it right.
I've looked up at the back of the control board and no LEDs are glowing.
The heater all looks unmolested inside -- no evidence of rats gnawing wires.

My gut is telling me to replace the control panel

 

[El-Guapo]

(oops -- I apparently hit "enter" and I don't see an "edit" button). My gut is telling me to replace the control panel, and I think I'm just going to order the control panel and the thermistor at the same time because the thermistor only adds ~$12 to the order. While I'm at it, should I be ordering anything else?

 

[JamesW]

Do you have a water temperature sensor in the system?

If yes, you can connect it in place of the temperature sensor in the heater to see if that makes it read correctly.

Do not operate the heater like that because you need a good temperature sensor in the heater for safety.

Just connect the water temperature sensor or air temperature sensor to the heater with the heater not operating to see if it reads correctly.

 

[JamesW]

Google search results

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Pentair Mastertemp 400 displays 126, already replaced thermistor

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...

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Led 7 is bad thermistor.

It is in the upper right next to the place where the thermistor plugs into the board.

SOLVED! Pentair MasterTemp 250 Error E01 - 126 -Not the Thermistor...

Hey gang. I am fearing that my control board has gone bad and before I take the plunge into an expensive replacement, I wanted to get some advice from smarter people than me on what else I could troubleshoot. -Would anyone by chance have a schematic for the board that shows logic flow? -I...

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Master Temp 400 E01/ 126 fault

Good day I have an E01/126 fault on my master temp 400. I measured the resistance across the thermistor terminals with wires unplugged and it read 8k ohms, I had ordered a new one and tried anyways and the same issue persists. I removed the brown wires from the control board that attach to the...

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[El-Guapo]

Do you have a water temperature sensor in the system?

If yes, you can connect it in place of the temperature sensor in the heater to see if that makes it read correctly.

Do not operate the heater like that because you need a good temperature sensor in the heater for safety.

Just connect the water temperature sensor or air temperature sensor to the heater with the heater not operating to see if it reads correctly.

Thank you for the reply. No other water temperature sensor in the system. I'm totally new to pools and spas, so I apologize if I sound clueless. I have been learning along the way. My system is about as simple as it can be. I have a newish Pentair variable speed pump, a filter, and the Mastertemp gas heater. If the kids want to use the hot tub, I go out beside the house and manually start the pump, then turn on the Mastertemp from the control panel. There's no remote control panel or display (which would be the only situation I can think of why there might be another temp sensor in the system).

I also saw the links you provided to other similar threads -- two of them say "E01/126 fault". I have the 126 on the screen, but it's not giving me an E01 fault. The system isn't giving me any error messages. It just isn't turning on the burner because (I think) it won't turn on the burner unless the chosen temp on the control panel > the apparent temp, and the system thinks the water is already 126 degrees.

So if the control panel says the water is 126 degrees, but my VOM shows that the thermistor is measuring the water at ~65, the control panel is the most likely suspect? I've already ordered an aftermarket control panel and thermistor online.

 

[El-Guapo]

Posting here with the resolution of my situation. My kid is happy. The spa is hot. Ordered a thermistor and a new circuit board from Amazon -- the two together were about $100. Before attempting any repairs, I checked the resistance again on the old thermistor and it was giving me around 13,500 ohms (around 65 degrees, which seemed about right). So I put in the new circuit board first, and....it gave me the same error (over temperature). So I took the wires off the old thermistor and connected them to the new (sitting on the ground) thermistor. The temp gauge showed 65. So it was the thermistor after all. I put the old circuit board back on, installed the new thermistor, and everything is working properly. Still mysterious: my ohmmeter and the old (bad) thermistor are sitting on my desk. I just checked the ohms with the thermistor in my hand. It started at around 9500 (80 degrees) and the resistance slowly dropped to around 8200 (85 degrees)as the sensor warmed up in my hand. So whether it is screwed into the heater or sitting in my hand, it seems to be giving me accurate readings. I still don't understand why the temp displayed on the heater's control panel was "pegged" at 126. Both the old thermistor and the new one were giving me similar values.

So I guess the moral to the story -- even if your thermistor seems to be giving accurate readings, it may be shot?

Thank you for your help!

 

[JamesW]

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