Here is the manual for all Atwood RV water heaters.
Barry is right on.
The red light in the Atwood DSI system is a "non-ignition" indicator; it will not illuminate unless after 8 seconds of calling for heat, the flame does not light.
In your old Suburban water heater, the DSI Fault light came on anytime the control board called for heat but the flame was not lit. Once the flame lit, the light went out.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
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HAM CALLSIGN - KC3FFW