Gen 3 Prius V - Hazards won't turn off

The Lead Up:
A few months ago, my 2013 Prius V with ~138K miles started to throw some errors where some of the lights on the dash went on indicating issues with the ABS system. I took a picture of them but I can't for the life of me find it right now. I think it was the lights for the Brake, Traction Control, ABS and a few others. The lights would come on and within 5-30 minutes would usually go away again.

The Problem:
About a month ago I walked up to my car after a quick stop, and noticed my hazards flash very quickly (<1 second) and turn off again. When I got in the car, nothing seemed to be amiss and the rest of the day went normally. The next morning, after another momentary stop, I came back to the car and the hazards were on. Since then, the hazards have never turned off.

I need to fix this problem within two months because I live in MA, and as part of the yearly safety inspection, all cars on the road must have functional turn signals and hazards, and my tag is due this month (they apparently allow a 60 day window).

What I've Tried:
I took it to a local shop who said that they thought that it was an issue with a relay in the dash, but that they couldn't get the dealership to get them the part and that I'd have to take it to the dealer. Thinking that this was a bit weird, I did a tiny bit of debug myself. I pulled the hazard switch from the dash and disconnected the cable and it made no difference. I pulled some relays and fuses hoping that I could isolate the fault by system, but no luck. The best compromise I've made is to just pull the fuse for the turn signals and the hazards. I still get the constant clicking (even with the car off) but at least people on the road aren't confused.

I took it to the dealer who diagnosed the car with several faults:

• C1239 Foreign Object is Attached on Tip of Rear Speed Sensor LH
  
• C1466 Rear Speed Sensor RH Circuit
  
• C1467 Rear Speed Sensor LH Circuit

Additionally they said that the fault causing the issue with the hazards was inside the ABS ECU as part of the ABS Brake Booster Assembly, and their solution would be to replace the whole assembly. In total they quoted $3k for the Brake Booster replacement and $1k for each rear wheel to replace the rear wheel hub and bearing assemblies.

I took the car home because I didn't want to pay them damn near the total value of the vehicle. I replaced the rear wheel hub and bearing assemblies myself because I'm at least moderately handy.

Finally, I got into the engine compartment and did a little fiddling. I found interestingly that unplugging the harness from the ABS actuator did not clear the condition. While I was in there I lightly scoured some of the contacts on the Brake Booster side of the connection in case there was corrosion making a problem, but that didn't help either. If the fault is caused by a short inside the Brake ECU pulling a signal to ground, like the dealership claimed, I would think that it's possible that unplugging the connection would clear the faulty condition. Because unplugging it didn't stop the hazards, it leads me to think that the short could be somewhere else like inside the harness.

Final Thoughts:
Do any of you have any suggestions on what I could try next to debug the system? It would be quite nice if I could do enough debugging myself to isolate the issue before I commit to spending $3k for the Brake Booster replacement if that's what it is. Thank you so much for reading this long post!

 

Thank you so much for your helpful replies! These EWD snapshots are just what I assume I need. Thankfully I'm used to electrical schematics and wiring harnesses since designing and making harnesses is what I do for a job! I'm still pretty ignorant about what's going on in this system, so therein lies the problem. And the constant changing of wire color is still pretty obnoxious.

I checked the battery, and assuming you mean the small 12V in the trunk, that one seems to be doing fine. I don't have a load tester, but I do have a DMM and the voltage doesn't sag under load to a worrying degree despite its age.

I pulled the switch panel out of the dash, and with the battery still connected I see +5V between 3(gray) and 5(Black/white) on connector L57 and no short when the battery is unplugged. That leads me to believe that the fault is either in the combination meter, or in he section of harness connecting the combination meter to the "Skid Control ECU" which I assume is the official name for the computer on the Brake Booster Assembly. Does this sound right to you folks?

rjparker, do you happen to have another EWD showing the connection between the Skid Control ECU and the combination meter? If you have a diagram showing its route through the car, that would be extremely helpful as well!

 

Update! After some more thinking, I realized that I'd made a rookie mistake. I unplugged the Brake Booster ECU again and hit the hazard switch and they went off. Sure enough the fault is inside the Brake Booster ECU. It turns out I have the same exact problem that is described in this post from 2021.

I've temporarily circumvented the issue by clipping the sky blue wire on pin 10 of the harness. I could have tried to trace the connection all the way behind the dash to brake the connection somewhere more convenient like a connector, but I honestly just didn't feel like it.


Once I plugged it back in, I can drive the car and use the turn signals again! If I get around to replacing the ABS module I can reconnect it with a splice. I might even try putting a diode or a high value resistor in there to keep whatever is going on inside the ECU from messing with the hazards.