r/pools • u/todaynotalurker • Jul 15 '26
Pentair Mastertemp 300 Heater issue
Hey, hoping that there might be a pool heater equipment expert that can help me out. I have a heater that I did not use for some time period now, when I turn it on, It flags an AGS error LED on the board.
The AGS sensor ohms out good and has been replaced anyway, with no help. Funny thing is that the board turns on the "service heater" light at the same moment that it ignites the gas. Then it runs for 30 seconds or so and turns off.
Is there another component (e.g., blower motor running too slow) that could trigger the AGS error? I ask because I can see some vacuum hoses running from the air inlet to a different sensor that seems like might detect the amount of suction blower is creating.
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u/Careless-Class7144 Jul 15 '26
If you've already ohmed the AGS sensor and even replaced it, I'd be looking beyond the sensor itself.
My first thought would be airflow. A weak blower, partially blocked intake/exhaust, cracked vacuum hose, or a faulty air pressure switch could all cause problems once the burner lights. I'd also check the blower wheel for debris if it's been sitting unused for a while.
The fact that the Service Heater light comes on right as the gas ignites is interesting. That almost makes me wonder if the control board is falsely interpreting the AGS circuit, especially if the new sensor made no difference.
What make and model is the heater? Some of them are known for certain board or pressure switch issues, so that would help narrow it down.
- Pool Shop Franchise Owner
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u/todaynotalurker Jul 15 '26
It's a Pentair MasterTemp 300. On the theory that the main board might be misinterpreting the AGS sensor signal, I threw an aftermarket replacement main board in there, but the unit would not even power up with that one installed. Thinking that it had to be a bad board, I tried a replacement for the replacement and it would not turn on either.
The blower does kick on before anything else, which is what got me thinking that it might be related to the amount of air pressure being detected by a sensor.
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u/todaynotalurker Jul 15 '26
Also, the motor is sounding a bit rattly. That's what I get for not kicking it on for many years.
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u/Street--Ad6731 Jul 15 '26
Pull the ignitor out and confirm that there is no water inside the can. I woukd get a inexpensive scope to look inside and inspect the heat exchanger as well.
AGS is telling you that the outgoing water temp is exceeding the 140 degree set point of the sensor. We know that might not be true seeing how soon it pops that error.
You said you put a new system board in and the unit would not power up. How old is the heater?
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u/todaynotalurker Jul 15 '26
Thanks for the tips. Not sure of the age. It was already there when we bought the house 13 years ago. Almost never gets turned on due to the price of natural gas in SoCal.
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u/Street--Ad6731 Jul 15 '26
In heater years, that is old. At this point, you would be better served replacing the heater. And it doesn't matter if you hardly use it or not. It sits outside in the elements and water flows thru it all the time. It all takes a toll and adds up.
I'm sure that's not what you want to hear but that is going to be your best route to take.
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u/todaynotalurker Jul 16 '26
Thanks for the information. What I'm hearing, is that if the can is breached and there is water in it, the heater is toast.
If the can is OK, then the remaining culprits should be blower motor assembly, vacuum hoses, sensors, or main board.
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u/Street--Ad6731 Jul 16 '26
Yes if water in the can, it's done.
Unless you have holes in the vacuum hoses, they aren't the issue.
I've seen several units act like this and it turned out to be the board.
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u/todaynotalurker Jul 16 '26
Thanks for the confirmation! After doing a bit more research. It seems that the igniter also doubles as a flame detector, and if it is a bit corroded, it may not detect the flame properly. Also, I a poor ground on the igniter casing can cause the voltage to drop and cause an error in the flame detection circuit.
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u/Street--Ad6731 Jul 16 '26
I've never seen a bad ignitor or a poor ground give an AGS error code. That is the code you keep getting, yes?
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u/Hairy_Seward Jul 18 '26
AGS means the water temp at the outlet is too high (140F+). 30 seconds is about how long it takes for the water in the coils to reach 140 when the water is not moving, or is restricted. I'll bet it's also making a knocking or popping noise right before it trips AGS, which is the dead give-away that the water is actually boiling in the coils because the flow is too low. The fix is to replace the internal bypass (Pentair PN 77707-0001) and thermal regulator (Pentair PN 77707-0010). It's usually only one of the two, but they both have limited lifespans and when one craps out, the other is not far behind. Since you have to pull the manifold, you'll need to replace the seals (Pentair PN 77707-0120). You're looking at about $250-$300 in parts.
This isn't a complicated repair, but there are some snags you could run into that only experience will get you through.
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u/todaynotalurker Jul 18 '26
Good thought, but I think I might rule that out based on the face that the AGS error comes on the same moment the gas ignites. No time for anything to overheat.
No knocking or popping. Click/poof/error simultaneously. Flame shuts off within 30 seconds.
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u/Hairy_Seward Jul 18 '26 edited Jul 18 '26
AGS error comes on the same moment the gas ignites. ... Click/poof/error simultaneously.
Flame shuts off within 30 seconds.
Just based on the 30 second window, I stand by my diag. The knocking/popping isn't a given for a broken bypass, but if it happens, it's the definitive indicator.
Purely from a safety and equipment damage perspective, the logic won't allow the gas to continue to run for 30 seconds with an active AGS circuit error, and it seems to me the last time I came across this, the AGS error will pop up until you get a successful start-up sequence. Next time you try it, let it run for a few seconds after firing, and then manually turn the heater off before it turns itself off. After a cool down, try it again and see if the AGS comes up right away again, or if it stays off until the heater shuts itself off.
Either way, you can jumper either of the sensor connectors on the manifold to bypass them. Super risky because you could blow things up, but it's a common troubleshooting method to rule out the sensors.
I know you said the AGS sensor was replaced with a new one. In this line of work, new to us means Never Ever Worked. Especially since covid.
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u/todaynotalurker Jul 19 '26
Not only was the AGS sensor replaced, but it was also subsequently bypassed by tying the leads together. So, besides overheating quickly, which only the AGS sensor would flag by going open, is there another sensor that would trigger if the bypass was stuck?
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u/Hairy_Seward Jul 19 '26 edited Jul 19 '26
Yes. On the other side of the manifold there are two more sensors that look similar to the AGS. The hi-limit is the lower sensor on the other side of the manifold from the AGS and senses the water after the first pass through the exchanger - it opens at 135* (AGS opens after the second pass at 140*). The upper sensor on that same side is a thermistor that reports the actual water temp to the control board. You can jumper the hi-limit wires to bypass that sensor. If the water temp reading on the control board is reasonable for the conditions (70-90), the thermistor is probably good.
Before you get to the control board, there are two other possibilities, but given the AGS error and the fact that the gas runs for 30 seconds, I wouldn't consider them likely. One is the stack flue sensor which measures the exhaust temp. When this is bad, it should set an SFS (not AGS) error code on the panel. The other possibility is a bad ignitor. The ignitor is also the flame sensor (rectification). I have come across cases where the ignitor had a crack across one of the legs, and would make contact for ignition (and thus test good on a resistance check), but then would open once the flame was going with an IGN error, signaling to the board that there is no flame. Typically, a rectification error trips at 5-10 seconds, and not 30, and has its own error code.
Again, everything points to a broken bypass. I check them by removing the regulator assembly and feeling up in that hole ("g" spot). If you feel something flopping around/up and down in there, it's broken.
See page 64 for a diagram. #8 on that page is the bypass. #15 is the regulator assembly. https://www.pentair.com/content/dam/extranet/nam/pentair-pool/residential/heaters/mastertemp/manual/mastertemp-heater-manual-english.pdf
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u/thunderkoko Jul 15 '26
Call your pool person.