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

3 months ago
last modified: 3 months ago

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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