Multiple Warning lights turns out of be U0129 fault code

Had a long thread looking for possible causes of sudden appearance of multiple warning lights. thanks to all contributors. I have finally devined a fault code and so starting a new thread to research what troubleshooting flow chart to follow for this code.

As a brief reminder I had Brake, Traction, Power Steering, Hybrid System and ABS warning lights all come on at once with no apparent loss of performance. Car starts and runs (and stops) but doesn't have speedometer or battery and engine power meter. (the battery state of charge meter is fine).

Regular OBD scanner yielded no codes.

I took it to a friend who had a shop style computer scanner who told me there was a brake communication error but he didn't give me an actual fault code number.

Dr. Prius gave me this which unfortunately is probably correct but wasn't recognizable as a standard fault code designation when I posted it in previous thread:

HV Battery ECU Error Code: CC129

I, like many others before me, fell in to the trap of thinking that HV stood for high voltage and there was something up with the high voltage battery independent of this brake communication problem when, in fact, I think Dr. Prius did find the relevant code but didn't report it in a recognizable way. This turns out to be a pretty significant confusion that folks believe the 'HV ECU' is high voltage ECU. Now of course it monitors the high voltage batteries and state of charge as well as many other factors because it is actually the mildly redundant "Hybrid Vehicle Control ECU", sometimes shortened to the less redundant "Hybrid Vehicle ECU" or abbreviated as "HV ECU"

So not realizing that Dr. Prius had got the relevant code, I took it to another friend who has the Snap-on Zeus and printed out the result which was:

HV ECU - Current Codes
U0129 Lost Communication With Brake System Control Module

(same is in History Codes)

Zeus could not reset the code or it the cord came back as quickly as it was reset. If that were the case I would have expeted the History Codes to get an added line for the repeat code but I don't know if that is how it would work so I'm unsure if it failed to reset or simply threw the code again immediately.

There is an excellent diagram of most ECU component locations posted in another multiple warning lights thread (albeit Gen 2, so I don't know if anyone has this clip from service procedures for the Gen 3). But it does not show the location of the "Brake System Control Module" nor explain why the convention doesn't refer to this as the "Brake System ECU".

My guess, given that the electric power braking and ABS all seem to handled onboard the master cylinder in threads I see discussing this, perhaps the Brake System Control Module is onboard or immediately adjacent to the master cylinder on the firewall in the engine compartment. But it is very difficult to see because the master cylinder is buried so deeply under the windshield with lots of stuff in front of it. I'm praying I don't have to get to it, as it looks like it would take just short of pulling the engine; but maybe someone can talk me off the ledge by explaining that it isn't as impossible to reach as it looks. Does seem like some parts of black plastic cowling around the windshield wipers could be removed for better access although a bit of a jigsaw puzzle.

The HV ECU shows in the Gen 2 diagram to be on the outside wall of the car passengers side adjacent to the glove compartment.

I don't see any obvious rodent intrusion and I've seen a few threads debunking the idea that the low voltage battery going low is going to set these relatively hard codes.

From my searching, this is a fairly general code (and utilitized in many Toyota and other manufacturer models from what I can see). Most of the threads I see where this code shows up in a Prius also return other C, P or U codes but the Zeus did not find any others. I imagine that techstream would be the gold standard to make sure I'm not missing any and i think I'm going to go with the V-Diag version; but , meantime, I'm looking for a U0129 trouble shooting flow chart so I can see if that code alone has any meaning and how to test for proper functioning of the components and harness involved.

thanks,

brian

 

on Dr. Prius it was listed as CC129 but i'm betting that stands for Current Code 129 and is a shortened version of the full code.which is U0129 as read on the Zeus. Don't know why Dr. Prius doesn't list full code. And it is confusing because there could be a 5 digit code starting with C but I don't believe this was a C code. Dr. Prius had it as HV Battery ECU but I'm pretty sure again that was bad transcription on their part and it is the same fault as U0129.

 

I see a fair number of threads with the lit dash and the U0129 but they have other more specific codes that drive the troubleshooting. I've got the U0129 all by itself.

This might have happened once for a little while 20,000 miles ago, we had some lights, can't remember if it was the same ones, but it went away quickly. This time I've had it for a month and a half so it ain't going away on its own.

I don't know if the troubleshooting checks come up in the techstream as well or that is a different resource.