Prius (2011) won't start / switch to "Ready" mode after 12 hours of not driving

First off, this forum has been extremely helpful in narrowing down what the issue is I'm having with my 2011 Prius. However, I've mentioned the problems I've assumed it is to my mechanic: 12v battery, brake sensor, and they still can't figure out the problem.

Here's the issue: This just started as it's gotten cold in the last month. I live in Texas, so it doesn't get that cold (~32 on coldest nights for a few hours). If I leave the car in the driveway for more than 12 hours or so and come back to start it up, the car will not switch to "ready" mode. The lights all turn on and it beeps, but never turns over to "ready". This thread shows a video of the exact start-up problem I'm having with the same model car, and she figured out that it was a 12v battery problem.

I took it to my mechanic yesterday and they've already racked up $300 in labor charges, seen error codes that the car sends, taken apart the wiring to test if it's loose, checked the 12v battery and say its fine... yet, they still are unable to fix the problem and can't replicate it now because the car will start if it hasn't been sitting for more than 12 hours. They offered to keep the car over the weekend so that they can figure out what the issue is on Monday after it's been sitting, but I am worried that I'll leave it there and just keep racking up a labor bill without a fix.

I've had the car for about 4 years (bought used from 1 owner). I have not replaced the 12v battery in that time frame, but the mechanic is saying it is not the 12v battery. I typically only drive the car about 25 mins a day, mostly city driving, and sometimes not at all. It's been on a few long road trips, and this is the first time I've had any issues with it.

I'm wondering if anyone else has any ideas of what could be the problem? Is there a way I would know that the 12v was the culprit causing this issue?

Thanks in advance for any help.

 

Thanks for your replies. I have an update: I just picked up the car from the mechanic and they were adamant about it not being the 12V battery... but I still have a hunch that that's what it is (bc it's a 6-7 year old battery, your recommendations). They found an error code (U0155) stored, but they aren't even 100% sure when this code was thrown.. it's just the last code in the system.

Here's what they said on the bill: "This fault could not be duplicated at this time but a fault code is stored that is likely fail point. U0155 fault code is stored in the vehicles control module (IPC). This fault indicates a faulty dash display and/or a failure in the vehicles can-bus communication system. The technician will need at least 2 hours to further diagnose the problem."

So I approved those two hours and this is what he found out: "The following morning no starting issues were present at this time. The technician performed a full pin out of the modules power and grounds and found no current issues. The technician then performed a test on the can-bus system to confirm proper operation of the vehicles can-bus communication system. Without the vehicle failing while it is here we will not abe able to confirm if the module has failed or is intermittently failing. The older prius (generation 1 and 2) have an issue with this and it is possible this vehicle may also. Due to the cost of the parts and time involved we suggest having the vehicle here while it is failing or leaving the vehicle here to re-test after sitting for 1 day or more."

Since the car is working fine once it starts for the rest of the day, I didn't want to leave the car over the weekend and rack up more labor charges. They are telling me that it is most likely the instrument panel cluster that is faulty and will probably need to replaced (~$2000 parts + labor).

After hearing their feedback, do you think it could still be a 12v battery issue? The guy I talked to said if it was the 12v battery that the car is "smart enough" that it will send an error code for that, and they only see the U0155 error.. Is that true?

I am going to replace the 12v battery in the next few weeks after I absorb this unexpected cost, but I wanted to be sure that this error code hasn't happened to anyone else with similar issues. Mechanic says this happened in Gen 1 & 2 prius where the IPS would overheat and they supposedly fixed it, but now seems to think this Gen is having the same problem. Doesn't make sense...

 

Thanks, I'd definitely replace the battery before investing $2k on something the mechanic isn't even sure is the problem It seems insane to me that the mechanic is telling me there's no way it could be the battery, even though it's the original 6 year old battery, and wants to blame it on something that would cost $2k to fix, even though he's not 100% sure about that either.

Or maybe that's not insane and just a sign that I need to find a new mechanic! I think the latter.

I did find this post that was similar, and again 12v battery seems to be the consensus on the cause.

Thanks for your help everyone!
I'm going to go ahead and replace the 12v battery either way first and I'll update this thread if I am still having the problem. If anyone has had a similar issue or any other ideas in the meantime, feel free to share.

 

I usually had to keep going through the startup process... turning it off and then pressing break in and pressing power button. It takes 4-12 times to turn on if the cars been sitting. Usually powers on immediately if its run within 2-3 hours.

I just went to do a 12v battery test, but seems like I may need the color dash display which my car doesn't have. Anyway I had recently driven the car within an hour but followed the instructions to try to test the 12v. Since I couldn't find the display on my model I gave up and went to start the vehicle. Wouldn't switch over to ready. Could it be because I started the car without the gas, further draining the 12v? I tried about 10 times and the car eventually switched to ready...

 

Sorry double posted