2005 Gen II Prius throwing P0AA4 Code

Those contactors are located inside the HV battery case. When first turning the car on, normally one hears three relay clicks from behind the rear seat. Those are the sequenced contactors that supply battery power to the inverter. One of them apparently has had the contacts welded closed, or else there is some foreign bit that has caused the contactor to no longer open. Hopefully it is not a corrosion issue.

To fix the problem one has to go inside the battery case and check on the condition of the contactors. They are located on the left side of the case in this picture:
http://techno-fandom.org/~hobbit/cars/batbox/73revisit.jpg

This means working around the high voltage of the battery, so one needs to be knowledgeable in this regard. Pulling the safety plug first is essential, then one can be unworried about voltage. If you list your location in your profile, more specific advice can be given.

 

how many miles on her? are you the first owner?

 

As post #2 indicates, the problem is a failure of one of the system main relays within the traction battery case, stuck in the closed position. This represents a safety hazard as the traction battery is supposed to be totally disconnected from the inverter electronics when the car is IG-OFF. That relay remaining closed means that high voltage potentially may be present at unanticipated places (depending upon electrolyte leaks in the traction battery case, some other wiring fault, etc.)

This relay is pretty easy to replace if you know how to deal with high voltage and can follow safety precautions in the factory repair manual, available at techinfo.toyota.com

If you do not know how to handle a high voltage repair, then let your local Toyota dealer handle the problem so you don't get zapped. Good luck.

 

Does the car work? Have you tried clearing the code?

The very best scenario would be the relay was once stuck, but freed itself leaving only the code. This is unlikely, but its worth a try.

If the relay is just lightly stuck you could rap on it with a rubber mallet. Pull the trunk liner out and you can see the battery assembly. The contactors live on the left (driver's side). Use a short piece of 2x4 to transfer the blows to the battery. Don't deform the battery and don't smash the orange plug. Clear codes and see if they come back. This to is unlikely to work.

Good luck.

 

I have the relays from my old 2004 battery if you need a set.

JeffD

 

??

The proper test would be to remove the relay, and measure resistance across the switched terminals, first with no power applied to the coil terminals; then with 12VDC applied to the coil terminals.

In the first instance you should see infinite resistance and in the second you should see 0.5 ohms or less. The relay which has failed will probably show low resistance whether or not power is applied to the coil terminals.

It sounds like you are measuring resistance across the coil terminals, which is of interest but does not address the issue at hand.

 

For my PC friends, $25 plus shipping. If you need a set, send me a PM.

JeffD

 

have you try to reset the car after the replacement? disconnect the 12v battery negative for about 10 seconds and reconnect. Then see if the code returns.

 

Jodi,

If you need a Battery ECU, I have one of those too.

JeffD

 

My spare ECU was working when taken out of service 2.5 years ago. How about $50 including shipping (it fits in the same box we used for the relays). I haven't tried to diagnose one before. I'll look at the repair manual.

OK, There is a PDF that fully describes the Hybrid system with suggestions on diagnostics/repair. It is almost 4 Mbytes so to send it to you PM me your email address.

Sorry that the relays did not solve your problem.

JeffD

 

I would be careful about publishing your email in a public discussion group (now edit it out) which is why I suggested using a PM. I sent you the manual section on Hybrid Control.

JeffD

 

What happened with this post. I have the same issue, same code. Relays not open to allow power thru. I don't expect it to be relay though. If you found the issue please let us know. Thanks

 

I have the same car and I'm getting $7e8 warning that the the neg contactor is stuck closed. What was the final outcome with yours?
I don't know how my car is not throwing a code, but just a warning. I think we need Holmes and Watson on this one.
Good luck. Hope all went well.

 

It will be helpful if you can post the actual trouble code. A trouble code is five characters and the first one is a P, C, B, or U. The $7e8 is just the address of an ECU that reported a code.