2007 Prius Inverter Failure?

Hi All, first post here after finding much information browsing the forum over the years I am having some trouble with my previously indestructible Prius & could use some advice.

Sequence of events for my trouble with my 2007 Prius with ~140,000 miles:

1. Drive home Tuesday night last week, no problems, car functioning normally as always.
2. Wednesday morning, I tried to start up but multiple panel lights flash on and off. Tried a few more times, but it stopped responding entirely & all panel lights went off. OK, I've replaced the 12 V battery before, so maybe I left the door ajar or lights on & need a jump.
3. Hooked up my battery charger to the jump terminal as I have done before & tried again. Panel lights flash on & off, some warning lights (sorry, can't recall which ones). Charger current indicator jumps up and down and can hear a relay clicking on and off inside the engine. No start. I give up and head to work in my wife's car as I am already late.
4. That night, pull the 12 V battery and check voltage. Very low, ~4 VDC. I hook the charger up and let it run overnight. Measures a little over 12 V in the morning.
5. Reinstall 12 V battery and try to start again. A bit more sane response this time, panel lights don't jump around but the triangle/exclamation point of doom is lit, and the smaller version on the LCD also. No start.
6. Got a tow truck to come out and take it into the shop. When getting ready to move it out to the street for the tow truck to load up, it starts up! Warning triangle is still present, but motors run fine and drives normally for the 30 feet. Truck was already there, so we loaded it up anyway and towed it to the shop.
   
7. Shop calls and says I need a new inverter, $4800 repair. Ouch! The car is only worth that much at best, so that is not really an option for me. The trouble code is P0A09, with sub-code P0A09-591. I'm bringing it back home today until I can figure out what to do next.

I researched that code a bit, looks like it may or may not be a failed inverter. Any opinions on what is going on, or additional checks to make?

I also found some information on replacing the inverter with a salvaged unit. It looks to be not horribly complicated, but wanted some expert opinion on whether that is something a part-time mechanic might tackle. I'm a mechanical engineer and have rebuilt motorcycle engines and some moderately involved car maintenance, so I am comfortable with complex repair projects, but no experience with hybrid electrical systems. I can get the protective equipment and tools needed from work, but don't want to get in waist deep if it's best left to professionals.

 

Thanks for all the fast feedback! Here's the latest.

• I believe the inverter cooling pump was a recall item. If that is true, then it has been replaced 2-3 years ago (I need to dig through my service records to verify).
• I picked the car up from the shop, it appears to work fine now (so far). It started right up, no warning lights of any kind, including the exclamation point triangle.
• Hybrid battery was fairly low, down to last 2 purple bars, but charged up to 2 bars short of full within the 2-3 mile drive home.
• Checked 12 V battery voltage via the maintenance mode menu, looks normal at ~14 VDC.
• Drove it out a few more miles to the store later, hybrid battery ran down a few ticks and charged back up a few ticks.
• Checked 12 V battery with a DVM, 14.1 VDC.

It seems odd that the inverter would completely conk out, them come back to life and perform normally and not generate any more warnings. I don't trust it enough to drive it the 27 mile trip down to work. Any other suggestions on what to check would be much appreciated.

 

Checked the battery this morning with car off after it was sitting all night, 12.55 VDC.

Took a gamble and drove it to work, 27 miles, no problems. Gas mileage was a little on the high side, 55.7 MPG whereas I normally get between 45-50, but traffic was light so conditions were ideal for good mileage.