Prius (2004-09) :: 2008 - Hybrid System Warning Again Even After Replacing 12V Battery

I haven’t been on these boards very often since getting my used 2008 Prius because until now it has been very kind to me in terms of not needing mechanical repairs. Unfortunately that good streak seems to have come to an end. Driving home Monday evening (after ~30min freeway driving), my master system warning light came on, accompanied by the hybrid system warning light. I didn’t know what the latter was – had to look it up – but I immediately pulled to the shoulder. The car was driving normally, however. Since shops and dealers were closed for the evening, after much debate I ended up having the car towed to a family friend’s body shop; they were still there and had a scanner to check the error codes for me. My car was on the side of the freeway for well over an hour with hazard lights on and the dome light on and off occasionally as we got in and out of the car. By the time the tow truck arrived, I discovered that the 12V battery seemed dead; car wouldn’t boot up all the way, wouldn’t shift to neutral and didn’t want to power down – I had to try turning it off upwards of 10 times. I was surprised, given just the hazard light and limited dome light usage. We jumped it, got it in neutral and on the truck.

At the body shop, they said there were no error codes related to the hybrid battery; something about transmission that wasn’t clarified, and errors for low voltage on the 12V battery. With everything off, it measured 10-something V; something higher when turned on, but they believed the problem was a dead 12V battery. I was surprised that it would suddenly give out in the middle of driving, but we went ahead and replaced it the next day. I picked up the car Tues evening, and had no problems driving it home, to work the next morning. Unfortunately, during my drive home – strangely at almost exactly the same location as the first time – the same hybrid system warning light came back on. I’d driving about 70 miles with it off at this point before the reappearance. This time I didn’t pull over to the side (had enough of that before) but rather drove a few more miles to a parking lot where the car has now stayed overnight. I note that the car seemed to drive perfectly normally during that time; no loss of power.