Check the Fenwal for an error LED.
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Can you show a video of the startup sequence?
If you are getting a "Service Heater" LED, then you will have an error LED on the control board or the Fenwal of the 5 button heaters.
Check the diagnostic LEDs or the Fenwal box LED for blinks.
Diagnostic LEDs are on the back of the older 5 button non-connected Pentair MasterTemp heater board.
The 6 button RS-485 Connected Heater displays the diagnostics on the heater display or IntelliCenter display.
Normal boot up is 888, 128 and then rX where X is the software revision number.
The reason that it displays 888 is to prove that the display works with all parts of the display.
Most likely, the display is missing parts and the code is a partial display.
Since it says "Check Heater" you will have an error on the back of the board or on the Fenwal.