Ok so I tried to tweak the priorities, but mine only go 1-4, and cargo rack, power plant, and discovery scanner can't be changed and are stuck on level 2.
You cannot set a module lower than one lower than the lowest other module.
What I mean is, if everythign is '1', and you change one module, you can only change it to '2'. Then, if you change a THIRD module, you can make that one '3'. Then a fourth to '4', and then a fifth to '5'.
Cargo racks and discovery scanners use no power and so it doesn't matter what priority they are so you cannot change them.
And obviously the power plant will never disable itself because it isn't generating enough power.