water heater works fine - IF- I leave door open

"The RETURN millivolt signal for flame is NOT that strong and will be disrupted by a slight short to ground." Return signal? Are you saying that the Suburban design uses the spark apparatus as double duty for whether or not the flame lit? If true, that seems dumb.

There does not appear to be any damage to the orange wire, looks new. The connection to the lug is shiny and clean.

As suggested, I cleaned the burner tube. The air-gas mixture is fixed, not adjustable.