A P3009 on a Gen1 Prius is a bit different than a P0AA6 on later Toyota hybrids in a couple ways. First, the P3009 does not have a sub-code to identify the area of the fault. When a P3009 sets, there could be an isolation fault anywhere: the inverter, the DC frame cables, the MG2, the MG1, or inside the battery case. All testing must be performed manually. Second, when a P3009 sets, it will not prevent the customer from continuing to drive the car. If a Gen2 sets a P0AA6 (the equivalent of a P3009), once the driver turns the car off, it will not start again until the code is cleared.
P3009 diagnosis can be broken into two sections: observation and application of logic, and then testing with an insulation test meter, also known as a “megger” or megohmmeter. Step one is to copy all the freeze frame data and then clear codes. Once the codes are cleared, energize sections one an a time to find the area of the leak. This will save time because you’ll have less to check with the megger. You must wait a few minutes after energizing a section because the code will take a while to set.
So, this car set the P3009 with the key on and READY off, so the leak is in the battery case. Next step? Remove the battery and check with a megger. Since this is a Gen1, it has the “plug to nowhere” which seemed like a great way to get inside the battery past the open SMRs. The meter reads 0.14 Mohm and that’s not enough, so we’ve confirmed our earlier test (or so it seemed). If you’re curious, a good battery will have over 550 Mohm resistance to ground.
Next I remove the cover and disconnect the SMR cables, the HV battery ECU, and the cables to the service plug. I’m doing this to make sure that the leak to chassis ground is not in the ECU, SMRs, or cables. I checked again and found a somewhat higher reading, but still not good. As it turns out, the HV battery ECU has 0.14Mohm resistance to ground, and that’s normal. So, if you want to condemn an HV battery ECU, you’ll need to do so by eliminating everything else.
It’s important to know that the HV battery ECU connector has nearly full battery voltage at all the time, even when the service plug is out. There’s not much danger in handling it, just like there’s not much danger if you were to unplug your 220V clothes drier. But if you try start probing or cutting into wires you could end up with some bad burns or worse.
The reason the high voltage is shorting to ground is electrolyte leaking from the battery terminals. If you look below the terminal on the left, you may see a pool of electrolyte sitting on the ridge and a lot of corrosion on the terminals. This battery is wet all over and there’s no point in trying to clean it; it’s leaking too much.
So, in summary, to find a loss of insulation on a 2001-2003 Prius:
Still want more information on high voltage loss of insulation? Check this out.