AGS and External Control questions for a Max-E-Therm 400 SR400NA heater

Nov 10, 2019
19
Palm Springs, CA
Summary of questions about problems I’m having with the heater today after trying to turn it on for the first time since March 2023:
1) Service Heater light turns on and AGS LED goes on before heater ignites. Just replace the automatic gas sensor plug or could other problems be causing this?
2) External control switch turns on the heater, but the display is still visible when the switch is turned to off when it’s in Service Heater mode. Is this normal?

Question 1: Starting the heater with a cold pool turns on the blower and the Heating light flashes. Within 15 seconds (but before the ignition fires), the Service Heater light comes on and Heating light turns off. About 5 seconds later while blower is still going, the Service Heater light goes out, the Heating light starts flashing again, and I get all excited at the idea the heater will work, but in about 15 seconds the SH light comes on and never goes out. Ignition never happens in this scenario, the blower eventually shuts off, and all is quiet. A check of the underside of the control board reveals the AGS error LED on. The heater is just three years old and the AGS plug is a very simple temperature sensor that would seem to be unlikely to fail in such a short time in operation, so that’s unexpected. New plugs are cheap (about 30 bucks with tax) and easy to replace, but I’m wondering if there’s more going on even though the AGS light is the only lit error LED. Water flow is more than sufficient and water temps are about 78 degrees right now so it’s not either of those problems. Any ideas of other areas to check out if the replacement sensor doesn’t work?

Question 2: I use an external three-way switch to launch the heater in either Pool or Spa mode (the “modes” are actually just different target temp settings). In normal operation, (1) when the external control switch is Off, the membrane display is blank and no lights are on, (2) when the switch is turned to Pool or Spa the display is visible and the heater boot up begins and the appropriate lights are lit, and (3) when I turn off the switch so it’s neither on Pool or Spa, the display blanks out. But when when I get the AGS error described in Question 1 , turning the switch to off doesn’t blank out the display. It seems stuck as if it’s permanently set to Pool or Spa. This is kind of handy when I’m diagnosing the Question 1 issue because I can check the error lights, but it’s weird because you’d think turning the external switch to off would kill the display. Pushing membrane buttons seems to have no effect in this state. The only way I can return to the normal off behavior is to turn off all power to the heater and then restore the juice. The display is then blanked out as expected, but it’ll go into the same frozen state once I turn the switch to Pool or Spa and the Service Heater light comes on. I’m wondering if this is an intentional feature, so that if the heater detects any sort of serious error and lights the Service Heater LED, the user would be alerted to the issue even if the external control is set to Off (presumably fixing the problem would return the external control to normal operation). Is that right or is there a problem with my control board?

Thanks for any guidance you can provide,

Nick in Palm Springs
 
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The max-E-Therm is the same as the MasterTemp in a different form factor.


AGS errors are usually caused by a bad internal bypass. Too much water is bypassing the heat exchanger internally. What water that is going through is coming out too hot and tripping the AGS switch in the manifold.

  • 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.
AGS error can also be caused by low flow.

When did you last clean your filter?

What is your filter PSI?

Do you have a VS pump?
 
Thanks for the quick reply! I should have been clearer that this is happening in a stone cold pool with heater ignition that never happens. As such, it can't be a bypass issue (no hot water to trigger the sensor). Though I do have a VS pump and keep it at only 1,600 RPM 24 hours a day, the issue I'm describing happens even with the pump at 3,200 RPM when set to Vacuum mode. Filter pressure is normal (for me).

I'll check the continuity of the sensor tomorrow. If the sensor is stuck open, I'd assume that the Service Heater light would be triggered immediately but it looks like it doesn't happen for about 15 seconds after the heater is booting up and then goes off for a bit before going back on permanently. I'd think a basic thermo sensor would either (1) work fine all the time, (2) not work at all, or (3) works but open at the wrong temp. What I see in the heater display suggests a sensor that works for a short time, doesn't work for a short time, works for a short time, then never works again. Weird. This smells like a control board issue...
 
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There is a specific startup sequence the heater goes through thst is described in the manual and Wiki. You can follow the sequence using a multimeter and see how far it gets and where it fails.

Diagnostics are more helpful than assumptions. Otherwise you end up throwing parts into ito it hoping you fix it.

The startup sequence is managed by both the PCB and the Fenwal box.

Does the igniter heat?

Does the gas valve open?

Do you get any puffs of heat during the startup?

@swamprat69 may be helpful.
 
Good advice. Besides checking the continuity of the AGS sensor today, I'll watch (and listen to) the startup sequence and see when the AGS light is triggered. The schematic ladder suggests that the AGS check doesn't take place until later in the sequence (if I'm reading this right).

To answer your questions...
Does the igniter heat? I'll check that today.
Does the gas valve open? I'll check that today.
Do you get any puffs of heat during the startup? No, because the burner never fires.

Thanks for that Wiki link... Very cool! Loved these instructions:
  • 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.
Brilliant...

Nick in Palm Springs
 
The schematic ladder suggests that the AGS check doesn't take place until later in the sequence (if I'm reading this right).

There is not an AGS check. This is an analog 20+ year old electrical control system.

The AGS sensor must be closed from the beginning of the startup. If the AGS opens at any time then the system shuts down.

The AGS is just a temperature controlled open/close switch.
 
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UPDATE:

The 3-year-old AGS switch tested with infinite resistance, which surprised me because of the fact that the AGS error LED didn't come on right when the system was first energized and then after it was lit, it went out again for a while before coming back on permanently. That sort of flopping back and forth suggested to me that the problem could be more complicated than just a bad switch, especially coupled with the behavior of the display\Service Heater light\AGS LED refusing to shut off when the three-way remote control was turned off. Worst case scenario would have been control board doing weird things. But like I said, the switch itself appeared to be bad, compared to a new generic one I bought for 27 bucks with tax and shipping included. That new switch tested between 0.1 and 3.5 ohms depending on how steady my hands were when holding the testing probes. Swapped out the old for the new and bingo, heater works fine and when I turned the 3-way remote to the off position, now the display and all lights blanked out as usual. I guess the heater keeps the display and LEDs on when the system shuts down with errors so you can see that there is a problem even when you aren't using the heater. Learn something new every day!

Thanks to AJW22 for talking me through this,

Nick in Palm Spring
 
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