Another.....Mastertemp 400 with AGS LED + Service Heater LED

Kihon

Well-known member
Apr 14, 2008
47
Huntington Beach, CA
Hello TFP experts.

I have Mastertemp 400 that's about 5 years old. I have attempted a a deep dive search here and found a few threads that very similar.
  1. MasterTemp 400 Heater won't stay running, but it was never confirmed if a new control board solved the problem?
  2. Mastertemp 400 - Burner shuts off, AGS and SERVICE HEATER lights
Does the same thing whether I turn heat on with my phone app (iAquaLink) or with the unit being in service mode(manual).
  • Cleaned filter to ensure proper water flow
  • Temp was set to 102
  • Revision 13
  • Thermister reporting ambient temp
The heater seems to be firing up normally when the controller calls for heat. The air blower spins up with the HEATING led flashing for about 10 seconds before the burner actually turns on and heat is felt coming out of the metal exhaust vent. However the instant the burner ignites, the SERVICE HEATER led turns on immediately. The burner continues to fire for 12--21 seconds before the heat/burner shuts off. Usually over 20 seconds. The heater is definitely turning on. I can feel heat coming out. I removed the top cover and checked out the LEDs. Found the the 'AGS' LED on...on the back of the board. Followed the troubleshooting chart in the install manual.

Purchased and installed new AGS switch from Amazon. Didn't fix it. Also at the same time, I replaced the thermal regulator. (tested old one and new one in hot water, both worked)

On the header, I noticed the bottom 2 bolts were rusted(plan on changing them all to stainless). Removed the 2 to see if any water game out. No water came out. Ended up keeping going. Removed the header completely so that I could check the Manifold Bypass Valve. Looked good. Copper on the heat exchanger pipes in and out looked good. Thermister and HLS looked good, but I haven't tested them. I haven't replaced the control board yet since it's pricey. Really don't wanna jump into the tub yet, since its a big job. Honestly don't wanna just replace everything in hopes to find a needle in a haystack.

What are some other tests I can run? I'm good with DVOMs, resistance checks etc.
Not sure where to do at the moment. Suggestions?

Video added:

Maybe one of the GURUS can help?: @JamesW @Arizonarob @ps0303
 
Last edited:

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
2,138
Stuart/FL
Hello? Is this forum dead?
K,
Welcome back! The forum is very much alive and well!! You've posted a fantastic video with way more information than we normally get to help. The experts you've called are probably out doing some last minute shopping. I'm sure they will be here soon and I'll add one @Jimrahbe more. He seems to know a LOT about Pentair equipment. I'd guess one of your thermisters is going high too quickly on you. The often fail that way. So initially it is low enough to "call for heat" then it thinks you're up to temp. The fact that you go through the start sequence to ignition and gas valve opens indicates your vent gas temp, hi-low sensors, and air flow switch are performing as intended. Could also be a problem with the flame sense circuit. I'll read up on your heater and get back to you but I bet one of the experts will solve before then.

Chris
 
  • Like
Reactions: Kihon

Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
15,884
Bedford, TX
K,

Sorry for the delay, but this is winter time for most of us, so not as many members around to answer questions this time a year.

I am not a heater guy, but if this were my heater, and based upon what you have done so far, the next thing I would look at is the Hot Surface Igniter (HSI). Does not sound logical at first as the heater is firing up... But...

After initially igniting the gas, the HSI then switches to the "sense" mode and starts to monitor the flame.. If it thinks the flame has gone out it will shut the system down.. The flame generates a little voltage in the HSI which the system uses to tell the system the flame is good. If these are anything like hot water heaters they can corrode or get discolored and no longer sense the flame. I have used steel wool several times to bring them back to life.

That is what I would try next.

Thanks,

Jim R.
 
  • Like
Reactions: Kihon

JamesW

TFP Expert
Mar 2, 2011
17,457
I would remove the leads from the AGS sensor and connect them together. Temp only for the test.

Then, check the continuity of the sensor. It should be closed. Start the heater. If the sensor opens, immediately turn off the heater.

If the sensor opens, it might be a bad sensor or the water is overheating.

To test the sensor, put it in hot water to see where it actually opens.

If the sensor does not open and the heater shuts off with an AGS light, the board is bad.
 
  • Like
Reactions: Kihon

JamesW

TFP Expert
Mar 2, 2011
17,457
We need to know if the sensor is bad or the board is bad or the water is getting too hot.

I understand that the sensor was replaced, but it can still be bad.

I would proceed with the tests as described.
 
  • Like
Reactions: Kihon

Kihon

Well-known member
Apr 14, 2008
47
Huntington Beach, CA
Hi! Thank you much for the replies! :)

Removed the AGS sensor wires and jumpered them together.

Started pump and heater. The AGS sensor NEVER lost continuity and the circuit remained closed all the way through the test. Heater came on and then went out off after 13 seconds. AGS LED lit on the back of the board.

Does this mean the board is bad?

I would remove the leads from the AGS sensor and connect them together. Temp only for the test.

Then, check the continuity of the sensor. It should be closed. Start the heater. If the sensor opens, immediately turn off the heater.

If the sensor opens, it might be a bad sensor or the water is overheating.

To test the sensor, put it in hot water to see where it actually opens.

If the sensor does not open and the heater shuts off with an AGS light, the board is bad.
 
Last edited:

JamesW

TFP Expert
Mar 2, 2011
17,457
If the wires are good, then I would say that the board is bad.

Sometimes mice get in the heater and chew on the wires. So, verify that the wires are good all the way to the board.
 
Last edited:

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
2,138
Stuart/FL
K,

As James indicated, if your wires are good the control board is probably bad.

I recently went through fits trouble-shooting a Jandy heater with similar controls. James, Jim and others on here were very helpful in tracking the problem down. My issue was totally different but I did find that there are weird things that can go wrong with heater controls and they're very low chance factor but I found ways to eliminate them with jumpers before I shelled out $300 for the expensive boards and modules. You seem to be pretty comfortable with internal tinkering so here's something you can check before you shell out the $300+ for the board. You can jumper from a 24 vac source to the "W" terminal on the Fenwal controller. This forces a "call for heat". In this mode the only thing that will stop the heat call is loss of flame rectification signal or a failure of the air flow switch signal. This essentially takes the control board out of the circuit. If you run in this mode longer than you have been it's a pretty good confirmation the problem is the control board. If not, something else (like one of those weird things) may be causing the failure. I've attached the Fenwal specification sheet for your reference. If you're not comfortable with the 24 vac jumpering then don't mess with this.

I hope this is helpful.

Chris
 

Attachments

  • Like
Reactions: Kihon

JamesW

TFP Expert
Mar 2, 2011
17,457
The board is indicating that it's shutting down due to an AGS issue.

By bypassing the sensor, it eliminates the sensor and overheating as the problem.

If the wires are good, that leaves the control board as the problem.

There might be other issues, but the board needs to be replaced first if the wires are good.
 
  • Like
Reactions: Kihon

Kihon

Well-known member
Apr 14, 2008
47
Huntington Beach, CA
K,

As James indicated, if your wires are good the control board is probably bad.

I recently went through fits trouble-shooting a Jandy heater with similar controls. James, Jim and others on here were very helpful in tracking the problem down. My issue was totally different but I did find that there are weird things that can go wrong with heater controls and they're very low chance factor but I found ways to eliminate them with jumpers before I shelled out $300 for the expensive boards and modules. You seem to be pretty comfortable with internal tinkering so here's something you can check before you shell out the $300+ for the board. You can jumper from a 24 vac source to the "W" terminal on the Fenwal controller. This forces a "call for heat". In this mode the only thing that will stop the heat call is loss of flame rectification signal or a failure of the air flow switch signal. This essentially takes the control board out of the circuit. If you run in this mode longer than you have been it's a pretty good confirmation the problem is the control board. If not, something else (like one of those weird things) may be causing the failure. I've attached the Fenwal specification sheet for your reference. If you're not comfortable with the 24 vac jumpering then don't mess with this.

I hope this is helpful.

Chris
Cool, i'm all for diagnosing it without tossing $300 parts at it. :)
Ya I'm good with testing 24vac supply to the W terminal. I'm currently a Sr Field Service Mgr for over 28 years, so I'm good with diagnosing with your guys help. :)

I did find some old snailshells inside the bottom of the 400(I Know rats eat snails), but I inspected all the wires and no chew-throughs that I can find.

Anyhow, My Fenwal doesn't have a "W" terminal. See this pic. thoughts? I've also attached the wiring diagram from inside the Fenwal box.
 

Attachments

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
2,138
Stuart/FL
K,

That's weird. As you can see the attachment I sent should cover your 35-66 series. I'll look around for an older version but I know it should have a "call for heat" terminal. That's the way every one of them I've seen works. I wouldn't want to guess as the modules are about $200+. I'm traveling today so it will be late before I can search, so let's see if @swamprat69 knows. He's got a LOT of HVAC experience and has been invaluable to me on the heater. Swamp, we're trying to find out what terminal is "call for heat" on his Fenwal. Would you know? Seems like most of the newer ones have a "W" terminal for this and the manual for 35-66 series shows that as well.

Chris
 
  • Like
Reactions: Kihon

JamesW

TFP Expert
Mar 2, 2011
17,457
The board monitors the AGS circuit to make sure that it stays closed.

Since the board is reading the circuit as open when we know that it's closed, it has to be a bad board.
 
  • Like
Reactions: Kihon

Kihon

Well-known member
Apr 14, 2008
47
Huntington Beach, CA

JamesW

TFP Expert
Mar 2, 2011
17,457
I'm not sure why there's still a question about the board.

If it's giving an AGS light, then it is shutting down the heater.

We know that the circuit is closed because the wires are connected directly.
 

bdavis466

TFP Expert
Platinum Supporter
Aug 4, 2014
5,293
San Clemente, CA
$65...Looks sketchy but I like risk :ROFLMAO:

 
Last edited:

setsailsoon

Gold Supporter
LifeTime Supporter
TFP Guide
Oct 25, 2015
2,138
Stuart/FL
Only reason I looked for ways to verify the results of standard trouble-shooting was I used them to verify my test of the replacement module I got. Even with manufacturer's tech support verifying I was doing everything right they said even new boards do show up bad and they recommended I replace. I did that and the replacement didn't work. As you know I found a weird thing that caused the problem. I've tested both modules and they both work great. So now I have an expensive spare. James, you're almost always right so it probably is a bad board but if there's a way to double check I'd do it. K, James is the expert so deferring to him is probably the right thing to do.

Chris
 
  • Like
Reactions: Kihon

JamesW

TFP Expert
Mar 2, 2011
17,457