Power button unresponsive in hot weather

Howdy y'all. I have a 2006 Prius with 143k miles, and it doesn't want to start in hot weather. I see the door light on the dash but nothing else comes on when I push the power button. No instrument cluster, no MFD, no noises. Is there a good place to start for troubleshooting? The hybrid battery and 12v battery are less than a year old. The 12v battery is currently showing 12.3v on my multimeter. Disconnecting the 12v battery for a few minutes will allow the car to start after hooking it back up.

 

Yes, when it's working everything comes on and functions normally.

 

That's what I thought too, but I didn't think the combo meter prevented the car from starting and driving.

 

It didn't though. I couldn't shift and it wouldn't move. Plus the MFD wasn't on either. I took a look at the negative battery cable and it doesn't look like the right size for the battery, it just has some shims jammed in the connector. I'm going to try to find the right battery cable and see what that does.

 

Fuel level was above half the last time it was driven.

 

I fiddled with all the cables attached to the 12v battery until I was reasonably certain all of them were making a good connection and it started up again even though the weather was about the same as it was yesterday. I'm cautiously optimistic that was the only issue. But I guess I'll see over the next week or so since sometimes it works and sometimes it doesn't.

 

Nevermind it wouldn't turn on again today. It worked fine for about a week after an hour drive and then it sat for about a week so I'm wondering if it just needs to be taken on longer drives more often. I charged up the 12v battery with jumper cables and that didn't do anything. The immobilizer light comes on when I push the start button and the key is too far away, and the door open light is on when the door is open, but those are the only two lights that will show on the dash. I don't hear any beeping when the door is open, but it does beep when the headlights are on and the door is open.

 

Yeah I was thinking about getting a new Toyota battery and a new negative cable. I just wasn't sure because the battery in it now is an Optima that's only a few months old. I found another thread on here where someone had the same problem and they had a loose connector in the wiring behind the dash so I'm going to poke around in there first before I drop a couple hundo on a new battery.

 

Although the battery measured at 12.8v after I charged it and still nothing came on so I feel like there has to be a wiring issue somewhere. I guess there's a chance the battery could still be defective.

 

That's the annoying part. Sometimes it will start multiple days in a row without an issue. And sometimes it just won't start even after disconnecting the battery. The headlights work fine.