Yes, if the "sub tank" fault light is illuminated the system will not work
Even after you fix the fault that caused the computer to set the "sub tank" light to reset that fault light you still have to either disconnect the battery, pull about 3 fuses or unplug the sub tank computer (battery is usually easiest).

I haven't ever pulled a sub tank switch part so don't know if it can be done and/or the globe replaced, Rodney (or someone else) may know.

When you get a chance can you also confirm both your fuel gauges are working OK and whether the "sub Tank" fault light is on or off