Calling all prius gurus. This is a head scratcher. Ready light only stays on for less than 1 secon

A broken wire with a clean connection doesn't count. Might need to trace those lines with a meter, maybe look for rodent damage.

 

Mud dobbers . That mud that they bill their houses out of like Adobe that stuff will go down inside of the slots in the fuse box It is like sand it is an abrasive so you need to make absolutely certain that your electrical contact cleaner or WD-40 blast into those slots where the fuse is go and pushes up the mud and grit out of those slots it's not just a spray clean and blow operation You might have to get a shop vac so you may want to use WD-40 as it's not flammable and things won't create havoc with the Shop-Vacs motor brushes sparking whatever and after you douse that real good have your Shop-Vac right there at the ready to suck that wetness and abrasive out of those slots If you push fuses in and feel the abrasiveness and the squeaking noise you haven't got it all that could be standing the fuse off enough to not make good contact just a thought mud daubers are no joke they can clog up a pressure washer wreak havoc on an engine all kinds of good stuff that mud you're looking at is brittle and breaks easily but boy does it stick and wreak havoc on almost anything I've seen $1,000 of equipment ruined by mud daubers just one summer from it sitting when I take my pressure washer out in North Carolina every summer they have made houses in the wands of my pressure washer on the intake piping where the water is going to go completely blocked I have to make sure I bring this equipment out spray it off with a garden hose first get the mud saturated and then put the nozzle on a thin blast just from the garden hose and make sure all that mud is gone so water can get into the pressure washer get pressurized and then squirt out the nozzle I've seen my pressure washer wand not able to push the thousand plus PSI water out of it just with mud daubers mud in the wand not even in the yellow or red or green click on tips just the tube of the wand It will keep that water from coming out and your pressure washer from working so make no mistake You have got to get all that stuff out of there. And then of course since you've had your batteries out and you're swapping them we know that there's no buildup or frost or corrosion on the bus bars and nuts? Then proceed from there

 

did you complete the 3 steps of the orange safety plug?

 

Check all your fuses, make sure EFI fuse is not broken

 

Those codes are because one of your ECUs isn't communicating... Once you've run out of wires to inspect, check for critters inside the ECUs and then start swapping them out with the ones from your other Prius. A used ECU if you need to replace it isn't very expensive.