Can only start by jumping under hood

Assuming you have a multimeter, with the car off:

1. Check that the electrical connections at the battery posts are clean, that the ground strap to the body is good, and that the thick cable from the front of the car is plugged into the gizmo on the positive post.

2. Make sure all is good with the 12V, battery and wiring wise:

Test 12V voltage at the battery posts.
Test 12V voltage at the jump point.

If everything is good the same 12.x V will be measured at both (plus or minus .01 V or so.) If it isn't something is broken between those two points. A likely culprit would be the fusible link in the gizmo on the positive battery terminal.

Jump the car to start and measure the voltage as above. It should be >13.5 V at both locations.

If all that was good, get a helper and watch the voltage at the jump point or battery posts while they car is turned on. If it drops below 11V the 12V battery may be very weak. It needs a load test. That is NOT the same as one of the little electronic testers, which use resistance and black magic to measure capacity. A load tester is just what it says, it puts an actual load on the battery. Alternatively, use jumper cables to attach a known good 12V battery in place of the one in the car (pull the gizmo completely off the positive post and carefully attach it to the jumper cable to the positive post of the good battery so that it cannot short to any part of the frame, the other jumper cable between the two grounds). Triple check that the polarities are right! See if the car starts like that. If it does then the 12V battery is no good.

3. Double check the safety latch on the HV battery. If that isn't seated properly the car will not start. P0A0D is the code. You may have a CEL, but the dash lights aren't working. I don't think you have this problem though, as it will start when jumped.

4. Lack of dashboard lights, if the 12V is good, is probably the infamous capacitor problem in the dash. There is a long sticky thread about it. When this is present it is really hard to tell what is going on with the car. Use a decent OBD2 dongle or techstream to pull codes and look at HV battery voltages and such.

 

battery tests aren't always accurate

 

The good news is this problem should have nothing to do with the HV system. The 12v system is what powers 'on' the car.

Just go step by step verifying the 12v circuit.
Good 12v battery
Good connections on the (-) terminal and the body fastener.
Good connection at the (+) terminal and both the heavy and light gauge wires fully inserted.
Proper voltage at the jump point under the hood and the cable fastener at jump point is tight. (that cable is the other end of the (+) terminal cable)

Your symptoms are typical for a weak 12v battery or a 12v circuit that has a bad connection.

 

Sounds like no connection at or near the 12v battery. Could be the positive or negative side including the 12v fuse at the battery terminal or the 12v battery ground terminal on the chassis.