Power at 12v but not at front terminals.

welcome!
if you're positive the negative to ground is clean and tight, i would try another battery just to eliminate the possibility. how old is it? even though it was replaced, i would check the date

 

You may want to take a look at this thread:

P0AA6, car won't start unless jumped, other codes | PriusChat

I would suspect you may have corrosion in the area of your System Main Relay, given that you have had water intrusion in the past, and the 12V does not seem to be able to connect to your HV battery. Before poking around in there, though, I would recommend that you take all the safety precautions necessary for working with high voltage, and I think it is essential that you get a hybrid compatible scan tool to see what codes are being reported before anything else.

 

So your checking the voltage on the battery fuseable link at the far end, not the end that connects to the battery and your getting full voltage? I think the only thing between that and the front jump point is the wire itself. Have you inspected the wire for damage?

 

you need a load test. auto parts stores will do one for free and give you a printout. otherwise, you can buy a load tester, or hook up a couple of headlight bulbs and watch the voltage

what brand battery? toyota truster have a date code, not a date

 

Interesting situation... If jump starting it from another vehicle causes the car to work fine and no error codes, then there's nothing wrong with the high voltage system, ECU's, Etc.

Easiest fix is if you bought an Optima battery because they're awful and I blame them for everything! But the symptoms don't really point to that.

As you go about checking fuses, you can also unplug stuff and look inside those plugs for greenish dirty / corrosion damage. Also for a few bucks you can by an auto test light where you plug one end to ground and use the other end to poke the connectors or cable to isolate where the 12v current first drops out of the circuit after it comes out of the battery.

Lastly, does this vehicle have a clean title or is it salvage from a flood / any signs of water damage?

 

The (+) cable from the battery goes directly to the jump point under the hood. Do a continuity check on the cable. If you're feeling crazy, temporarily run a long heavy gauge wire from the battery + terminal to the jump point and see if your problems go away. 12v+ at the battery post and 0.5v at the jump terminal just means you have an open in the circuit somewhere.

 

I thought that too, but wouldn’t Toyota put some kind of fuse at both ends? If the cable failed while the car was running the wire would be live from the jump box to the failure point, not good. The wire is a good idea. At least you will know it the cable from the battery to the box then.
Edit: I forgot about the 100 amp main in the fuse box. That would cover that end.

 

I have a complete wire harness from a disassembled 2007 in my garage. One end of the cable attaches at the 12v battery + terminal (in actuality, it attaches to the black plastic assembly, which in turn clamps to the post) and the other end attaches directly to the jump terminal. Nothing in between except copper and insulation.

 

So the other fuse is the 100 amp in the box then?

If he is measuring 12v at the cable end of the battery fuseable link and 0 at the jump points it has to be that cable. 100%.

 

@BobThePrius show us a picture of where you are testing the 12v at the battery.

 

here's what I would do (with no tools required other than a meter)...

With black lead on (-) terminal of battery and red lead on battery (+) terminal...should be 12v. If not = dead/bad battery.
If it is 12v, then....

With black lead on body of car and red lead on (+) battery terminal...should be 12v. If not = bad (-) cable or connection.
If it is 12v, then...

With black lead on battery (-) terminal, poke the red lead through the 12v (+) cable insulation near where it comes off the battery fuse holder assembly. Should be 12v. If not = bad connection or bad component at the (+) terminal, or between the terminal and where you touched the cable with the red lead.

If it is 12v, then...
measure with black lead on body of car and red lead on the under hood jump terminal. Should be 12v. If not, verify your black lead is correctly making contact with the chassis. If so, then the cable has an open in it somewhere between the previous 'poke' and the jump point.

 

I can’t tell from the picture, are you checking the voltage at the big wire or from of the white connectors?

 

it seems to me that if you have voltage at the neg terminal, but not at the ground, your whole problem is between the two
think you're gonna have to bust that bolt and start over

 

You cannot put the black lead on a painted surface, which it appears to me you are doing when you get the zero reading after checking the (-) cable where it attaches to the car body.

Put your black lead on the bolt head I placed an arrow near. That bolt should be plated metal and will conduct. Use that as your ground point/black lead for all your measurements in that area.

 

I don’t know if that will show if the fusible link is bad or not. That connector powers the brake capacitor box (that black box). That might have its own fuse as having brakes is considered important by Toyota. You need to check the main wire itself, no other wire or connector will do.