GEN 3 (2010) P0AA1 contactor code

It sounds like one of your contactors back under the battery cover is bad it happens I wouldn't say it's not rare and that's that those things are activated I do believe by the 12 volt system when you push the button up front it sends 12 volts to the contactor which then opened both of them sending HV battery voltage up front to the back of the inverter etc It's just like your air conditioner contact or relay going bad same thing I doubt any grounding has changed unless you're having other battery codes which is not mentioned here so probably maybe just a relay the Gen 3's that I've looked at under the hood of the battery all were pretty corroded so there's always that lift your battery cover look under the plastics and see how bad your bus bars and nuts look It is a thing.

 

If you care about the nits and grits you can wade through a search like relay vs contactor - Google Search but it's generally enough to say a contactor is a honkin' heavy-duty relay.

You've got three of them back inside the battery: a big one on the positive side, a big one on the negative side, and a smaller one (called the precharge contactor) that has a resistor in line with it and goes around one of the other two.

These are the things responsible for safely isolating your high-voltage battery when the car is turned off. The car switches them on and off in a careful sequence, watching what the voltages do, so that it can detect if one of them is stuck closed (which means your high voltage is not getting safely isolated when the car is off).

Don't override or bypass them, or crack them open. They handle high voltage and upwards of a hundred amps of current. If one is stuck, it should be replaced with the right thing from Toyota.

The repair manual will have a section on P0AA1 with troubleshooting steps to make sure whether the problem is really the contactor or possibly something else:

Toyota Service Information and Where To Find It | PriusChat

This is a case where going right to the manual online is probably worthwhile. People often look in a PDF of the manual that can sometimes be found on the web, but that one is missing all the detail sections for codes from the hybrid system, so your search for P0AA1 will only land on the brief mentions in the DTC chart and not the full troubleshooting sections at all.

At least from the entries in the DTC chart you can see there are two different INF codes that can go with a P0AA1, but again, you'd have to look in the non-incomplete manual to find out exactly what those tell you.

 

The small one is the precharge contactor. The two big ones are the positive and negative.

When the code gets cleared, that erases the car's record that there is a problem, so it will allow another start. Because it detects a stuck contactor through the clever sequence of switching it does when powering up and down, it doesn't rediscover it still has the same problem until it has gone through that cycle.

There's nothing in any contactor that would "reset itself"; they're just big dumb electromagnets that pull contacts together.

Be sure to check how the parts are sold at the dealer before getting the bigger tools out. If the only part they sell is the battery junction block assembly, you don't gain much by pulling that further apart (and depending on how it does or doesn't come apart, you could wish you hadn't).

 

You should be able to go to any of the battery rebuilders in any town and they'll have the stacked up they might even give you a set of relays just depends on who you're dealing with they should be plentiful I don't think it's a big thing these going bad a lot. When you start your car you hear the sequence and if it sounds different you need to find out why That's what all that clicking is you hear in the back and it's a very specific sequence If it changes one of them probably isn't working for one reason or another it's just like a switch.

 

Usually you buy that black block with the relays in it as a whole assembly I haven't looked at it close enough to see if you can literally take the relays off of the block doesn't really interest me that much I'll look this afternoon I have several here I do believe and yes you have a contractor for the positive side of the 211 v and neg side. I had no idea they would separate off the plastic block or the assembly but I've never tried to fool with that I just moved the whole assembly over and throw the non-working one out they're hard to find in the junkyard if you wanted to find these as a part you would call a battery reconditioning place like green bean or the guy the advertises on the marketplace in your market that rebuilds batteries he'll have these cases stacked up ready to go to the dump that sort of thing from these guys you can buy wiring harnesses pieces and parts they have the stuff just laying around they are constantly having to make trips to the dump to get rid of this stuff The guy I've dealt with in Virginia has so much stuff on the side and behind the little shop he rents pretty soon the cases and case parts are going to be a bigger pile than his whole shop is I don't know how he gets this stuff out of here but apparently not very easily.