Originally Posted by MtnGuy
I have no idea why the gas portion of the water heater would not work while on electric. Those are supposed to be 2 separate systems.
If the electric heating element already has the water heated to temp, then there will be no need for the propane/dsi to light, since the thermostat won't allow it as the water is already heated.
However, if you are taking a shower or using a lot of hot water quickly, then once the water temp inside the heater falls below the certain set point, then BOTH sides thermostats will allow their respectively controlled sides to come on.... the electric heating element....and if you also have the propane/dsi switch on, then the propane sides thermostat will allow it to fire up simultaneously too.
Usually though, if you have both sides on, and the water temp is just basically being maintained (not taking a shower, or using a lot of hot water), then the electric heating element is what will maintain the water temp inside the heater.
As stated the light (which is actually a fault light), only illuminates if the propane fails to ignite, or is at the very first of the ignition cycle, when you first turn the switch to on, and the DSI attempts to light the propane. (provided the water inside the heater needs to be heated when you turn the switch on).
This FAQ may help:
Suburban's electric switch and much more
It's very possible, it's all working correct, and you may not have any problem.