Prius C stalls and later shows "Check Hybrid System" error

I’ve found a lot of great information on this forum over the past few years, but now have a problem that has me stumped and am hoping someone can lend some insight. I apologize for the length of the post, but wanted to include plenty of detail of what I’ve experienced and tried.

First, the short version:

I have a 2012 Prius C with ~120k miles that’s currently showing the “Check Hybrid System” warning. On several occasions the engine has stalled when at a stop and there is no power from the electric motor. One battery block occasionally reads up to 2v below the other blocks. Could one bad module cause the engine to stall like this?

Full background:

When I first got the warning I was driving in EV mode through a park when the engine kicked on and the “Check Hybrid System” warning came on. I unplugged the white wire from the fuse box for a couple of minutes, and then the car operated normally for a couple of weeks.

The next time it had an issue was when I was pulling into a very slightly inclined parking space after a ~25 minute drive. As I came to a stop, the engine felt like it stalled, though I was turning the car off and didn’t think much of it. A little over an hour later I got back in the car, put it in reverse, and the car only coasted back out of the parking space with no power. The engine would attempt to start, but it wouldn’t turn over, and there was no power from the electric motor. At this point I was on level ground and tried unplugging the white wire again (I didn’t like the option, but was over an hour from home at that point). The engine started up and I drove off. I was low on gas at the time (one or two bars I believe), so my first stop was the gas station where it acted normally. The drive home was normal until the last half mile. The car had been in EV mode on the residential streets, but right before the house the engine restarted and the “Check Hybrid System” warning finally came on (there had been no messages since the first one weeks before).

I did some reading on here and found some potential causes, that would need to be done at some point anyway, so I started there. That included changing the spark plugs, air filter, and oil, checking coolant (levels which were fine), and cleaning the throttle body, MAF, and battery fan under the rear seat. I also removed and reinserted the orange battery disconnect. After all of this, I took a test drive around the neighborhood and at first all seemed good. At one point I stopped at a stop sign for a couple of minutes (very quiet neighborhood), and the engine stalls while stopped. The “Check Hybrid System” message came on and there was no power from the electric motor. The only way I could get the car back to the house, short of a push or tow, was to, again, disconnect the white wire for a couple of minutes. After that, the car ran normally for the quarter mile back to the house.

I had an ODBII reader in another vehicle, so I connected that and added the Prius C codes to Torque. It registered the P0A80 code. I drove around the neighborhood for a bit to get some readings and the three temperature readings didn’t seem terribly high. The 10 voltages were all generally in the 14v - 17v range and the difference between the highest and lowest readings was less than 0.5v with none of the blocks consistently being the highest or lowest.

I then took a longer test drive while logging data, and, while at a stop, the engine stalled and the car had no power, but the error message did not come on. I turned the car off and it then restarted and ran normally for 8 minutes. Upon approaching the house, in EV mode, the engine kicked on and the “Check Hybrid System” message reappeared. In reviewing the log, I’ve found two periods where battery block 5 dropped to ~2v below the other blocks, and these two instances coincide with the stall and error message. During that drive, the other blocks fluctuated between 13v and 18v, but block 5 dropped down to below 12v in those two instances. Other than these two instances, the block all were within 0.5v of each other or less.

My thought is that this indicates that one of the modules in block 5 has gone bad, but I would appreciate any input since I’m uncertain if a bad module would cause the symptoms that I’ve been experiencing.

Thank you very much!

 

Thank you everyone! I already have the back seat taken out, so I'll try the Techstream software and look at the rebuilding threads in more detail until the cable arrives.

Is there a particular version of the cable/software that I should be looking for, or avoiding?

I'll try to explain the "stalls" a bit more. It's distinctly different than the normal shutoff of the engine. In those cases, the engine was running fine while at a stop and then there was some hesitation before the engine turned off, that's consistent with what I've experienced in other cars when there's a lack of spark or fuel (probably a bad example, but think lawn mower running out of gas versus being turned off). I'm rather puzzled by how this could relate to the HV battery, though I believe one of the stalls was accompanied by a message involving "traction" but I didn't make note of the message at the time and it did not reappear after turning the car off, nor did not come up with subsequent stalls. At the time, I hadn't realized that the HV battery was also referred to as the traction battery, so it didn't make much sense...