Short version: A 2007 Prius with 200,000 miles kept setting P0AA6 — a hybrid battery voltage isolation fault — after a reconditioned battery had been installed. Testing with just the ignition on isolated the voltage leak to the battery itself, and during removal we found corrosion in multiple locations, including the computer terminals. A new OEM Toyota battery fixed it.
| Code | What it means | Most common cause we see | Typical fix |
|---|---|---|---|
| P0AA6 | Hybrid Battery Voltage System Isolation Fault — the insulation resistance between the high-voltage circuit and the vehicle body is too low | In this car, a failed reconditioned HV battery with corrosion on the terminals and battery ECU connections | New OEM Toyota HV battery; the battery ECU was replaced with a rebuilt unit |
A 2007 Toyota Prius with 200,000 miles. The owner already knew the code: P0AA6, a voltage isolation fault in the hybrid battery system. They’d been clearing it with their own OBD reader and driving for a while, but it kept coming back. When it did, the red triangle would appear and the car wouldn’t go into drive.
What we found
The car wouldn’t ready up, but initially no relevant codes were stored. Short Wave Highest Value (SWHV) was reading about 3.5 volts with just the ignition on. It should sit around 5 volts when there’s no leak present. We ran the three-step isolation procedure, and during testing multiple warning lights came on. Code P0AA6-526 set: Hybrid Battery Voltage System Isolation Fault, meaning the insulation resistance between the high voltage circuit and the vehicle body was too low.
We ran the test a second time with just the ignition on. SWHV dropped below 1 volt. That confirmed the HV battery was the source of the leak. The battery had a sticker from a reconditioning shop on it. These reconditioned batteries are generally a temporary fix and don’t hold up long term.
The owner had a good question. The car had previously thrown a 613 subcode, which points to a voltage leak in the transaxle. They were worried about spending $3,000 on a battery and still having a problem. We addressed this by looking at when the fault appeared. With just the ignition on, the transaxle and inverter aren’t part of the circuit. SWHV still dropped during that condition, which isolated the fault to the battery itself.
The fix
We replaced the HV battery with a new OEM Toyota unit. During removal, we found corrosion in multiple locations on the original battery, including the computer terminals. The battery ECU was replaced with a rebuilt unit.
After reassembly, we monitored SWHV with the ignition on for about 15 minutes. It held steady at 5 volts and never dipped. Road tested a few miles with no issues. SWHV stayed right where it should be.
What this means for you
P0AA6 is one of the more common hybrid battery fault codes on the Prius. If you can clear it and drive for a while before it returns, the battery is failing intermittently. It will get worse.
Reconditioned hybrid batteries can work for a while, but they’re a gamble. This one had corrosion on the terminals and inside the computer connections. If you’ve had a reconditioned battery installed and you’re starting to see P0AA6 again, the reconditioning probably didn’t hold.
If your code includes a 613 subcode, that technically points to the transaxle. But the same code can be triggered by the battery or inverter when the drivetrain is running. Testing with just the ignition on — where the transaxle and inverter aren’t part of the circuit — can help narrow down whether the battery is the actual source. In this case, SWHV dropped with just the ignition on. The transaxle wasn’t involved. Properly assessing the cause of P0AA6 requires specialized equipment and training.
Read the full guide: Toyota Prius & Hybrid P0AA6 Code Explained — what P0AA6 means, the common causes, and why it is often not the battery.
Related reading
- Toyota Hybrid Trouble Codes
- Toyota Prius & Hybrid P0AA6 Code Explained
- 2008 Toyota Prius P0AA6 High-Voltage Leak Diagnosis
- 2009 Highlander Hybrid P0AA6: How a Leaking Sunroof Almost Cost a Hybrid Battery
- Do I actually need a hybrid battery replacement?
About the author: Travis Decker is the owner of Atomic Auto in Portland, Oregon, and an ASE Master Technician (L1, L3). Atomic Auto specializes in Toyota, hybrid, and EV service.