MasterTemp 400 won't stay lit

Do they have the older model or the newer model heater?

Since his old board did not have the red/black diagnostic connection to the Fenwal I am assuming the old board. Otherwise he would not have asked about that wire.

The new board works in the old heater with the new keypad but does not have the new RS-485 and diagnostic messaging functions.
 
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The red/black cable only connects to the new model Fenwal that is on the "connected heater". If you have the old model MasterTemp heater there will not be a connection on the Fenwal for the diagnostic digital data.

Check the seating of the keypad connector on the board. Or you got a bad keypad with your new board.
Fairly sure I have the older model. My previous membrane/keypad had five buttons. I've attached a pic of my old keypad but doesn't look like I have one of my control module. Will have to grab one tomorrow.
 

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@JamesW I'll get some pics of the control module tomorrow. I'm nearly certain it's an older model. You an see the pic of my previous control board's membrane above and it has five buttons.

It's weird. The new board actually seems to be mostly working, along with the buttons on the membrane. I can turn the heater on/off, and the temperature buttons also seem to work - when I press up, the heat will kick back on. Just ran it all for about 45 minutes at various pump speeds and everything worked great, even with my pump in "quiet" mode. Spa got nice and hot. It's just that nothing shows up on the LCD besides the backlight.
 
@JamesW I'll get some pics of the control module tomorrow. I'm nearly certain it's an older model. You an see the pic of my previous control board's membrane above and it has five buttons.
You have the older style. The connected heaters (and new PCB/membrane you now have) didn't come out until 2021 so anything earlier will be "old".
Visually they're easy to tell apart too, as you've described. 5 -button = Old, 6 button = New.

I've attached the sheet we got from our rep last year.
 

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It's weird. The new board actually seems to be mostly working, along with the buttons on the membrane... It's just that nothing shows up on the LCD besides the backlight.
There should not be a compatibility issue, so I'm thinking you either have a lemon or it needs a power cycle. We have had them do this to us before and a power cycle would bring them back.

Was your product new and in the original package? I've attached a photo of what it should have looked like.

PN 461105
 

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Since his old board did not have the red/black diagnostic connection to the Fenwal I am assuming the old board. Otherwise he would not have asked about that wire.

The new board works in the old heater with the new keypad but does not have the new RS-485 and diagnostic messaging functions.
That's what we expected as well! But our experience has been that the new PCB will report at least some errors whether on an old or new model. We've seen PS, E06 and IGN Errors on older heaters we've upgraded with the new panel. And in all cases its been as accurate as you would hope. Interestingly, on the old board E05 was consistently SFS but is E06 on the new; in fact a heater that regularly threw EO5 on the old would throw E06 on the new as soon as you made the swap if you did it before the SFS was replaced.

All this said, I'm sure we are missing data from the ICM but it's at least producing some 'helpful' data as an upgrade!
 
Seems like a bad circuit board.

There is an LED on the Fenwal and not a connection terminal, so you can't use the red/black cable.

I would assume that if you removed the LED and connected the cable directly to the terminals it would probably work as it is probably just reading the pulses that would have gone to the light.

However, this is not equipment that you modify due to the safety ratings associated with the equipment.
 
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