Code P0AAC... Help!

does it mean one of your cells is overheating?

 

This means there is a fault in the circuit leading to that temperature sensor, either an open or a short. If that is the only DTC logged by your car, then I suggest contacting the supplier of the traction battery to request a warranty replacement.

You can drive the car indefinitely with that issue. The only part that might be hurt is the traction battery, if it actually starts to overheat the traction battery ECU will not know.

 

The cool code machine should have a screen to display live data. Or just click on the snowflake icon next to the P0AAC code to see the freeze frame data. There are three temp sensors clipped to the bottom of the battery modules. They are called A, B, and C. They should all read about the same temp. Sensor A is not. A likely cause is that sensor A has become unclipped from the battery module. Or was not clipped in properly during the rebuild. Yes, I learned this the hard way just as you are now.

You have a one in three chance of getting lucky. One sensor is on the first module next to the battery computer. The second sensor is smack in the middle of all the modules. The third is on the last module right next to where the air duct connects to the battery. I do not know which is A, B, and C but it is a good guess that B is the middle one. Wait, that makes your odds 50/50. Remove all the trim and carpet and such from the passenger side of the battery. Of course disconnect the 12 volt battery and pull the orange safety plug. Once you get the lower air duck out you can see that sensor with the battery still in the car. A mirror might help to see into the space under the modules. If that sensor is hanging loose just clip in back into place. If it is the sensor on the opposite end of the battery then you will have to get much more up close and personal with high voltage. Ask and know what you are doing before taking the cover off the battery.

Brad

 

This is where Techstream is more useful than a normal diagnostic computer.
Look at the battery temperature readings in the lower-left corner - TB1, TB2 and TB3.
TB1 and TB2 are ok (and almost the same) but TB3 is showing a negative number.
Sure enough, the clip was broken for that sensor.
That wiring harness is not available from Toyota. Luckily, I had a spare one, and now that this has happened once I'll make sure that I keep spare harnesses in the future.
If anyone needs one of these harnesses then search on eBay for "Prius Hybrid Battery Temperature Sensor Harness".