Occasionally no power to start

Hi. I hope someone can help me. I have a 2015 Prius 2 with 72,000 miles. I have had both batteries checked twice (and both are fine), but I still occasionally have absolutely no power when I try to start the car. However, once the car is running, everything runs perfectly. I'll try to post a video of my trying to get it started (with some audible frustration as I had 5 hour drive ahead of me that day).

It's not a key proximity issue nor a battery issue, and I'm certain it's not something draining the battery like leaving a light on in the car. The times it has happened have been after being parked for 10 minutes, 3 hours, and overnight. I get in the car, depress the brake, press the start button, and nothing at all comes on in the dash. Sometimes the start button light turns green and often there's a hum/rattle in the background, which changes pitch at times. One of the times the dash showed a check engine light faintly and briefly, but it was checked out at a Toyota dealership (before the time with the check engine light) and they couldn't find anything.

The really strange thing is that after trying the start button numerous times when it isn't working, I walk away and don't do anything for 5-10 minutes, and then I try to start it and everything works perfectly. One time I left the hum/rattle going (it seems I can stop that sound by pressing the start button a few times), and then the car started normally after 10 minutes. Another time I just walked away from the car and came back 10 minutes later and the car was already on! (without the key in it) So I must have hit the start button in some sequence to be registered as on but with no power to start it...and then something charges up to get it going. It makes me wonder if the hum/rattle has something to do with a starter (but I don't know cars, especially Prius).

And by the way, I sometimes hear the hum/rattle for a few seconds after I shut off the car, and also when I unlock it before I get in and start the car, which I always found strange.

Apologies for the length. I'm just not sure what to do at this point. I'll try to load the video.

Thanks!
Peter

 

Thanks everyone! I checked the positive and negative terminal and ground connections on the 12v battery in the trunk and they are all very clean and tight. And the batteries were checked by the dealership last week and were OK, but I don't know what else they did besides look for check engine codes and there were none. Any other thoughts?

And for reference, it happened again this morning, so the issue seems to be more frequent now. However, I left it alone for about 3 minutes and then it started right up with full power.

 

I have been DIY'ing to the best of my ability and have spent many hours looking up similar issues and trying various suggestions. The only potential fix I've come across that has worked for others with a similar issue is what was suggested here with checking the negative terminal cable and ground connection, both of which are impeccably clean and tight in my car. With an independent garage AND a dealership both having checked both batteries and not finding a solution to the issue, I've turned to the valuable resource that is this forum, and I thank everyone who has taken the time to respond.

I should add a few things too...
- When the car does start after having the no power issue, the battery indicator on the dash shows a good 80% charge, so it's not like it's been fully drained and contributing to the problem that I'm aware of.
- I've made sure all internal lights are shut off, including in the trunk.
- I'm always locking it once parked and have not heard the beeping that would remind me that something might still be on.
- The times the issue has presented itself seem random - 1) once after driving 3 hours and leaving it off for 3 hours; 2) once after a 5 hour dive and leaving it off for 2 days, it then starting fine and driving it for 1 minute, shutting it off and having the issue after 10 minutes away (you'd think the problem would occur directly after being immobile for a few days); 3) twice in the morning but got it started after a 5-10 minute wait; and a few other times.

Hope this helps. Thanks again folks.

 

And I'll add that the clock time and mileage are retained once the car starts again, as well as radio stations, but the fan/heating/cooling is shut off (when it was on before I last left the car).

 

Thanks. I haven’t checked it in the morning but have been thinking along the same lines...bad battery. I got the car used a few years ago so don’t know if it’s original. It’s now at the dealership again as I’m in desperate need of having transportation. And I’ve communicated all these thoughts to them and showed them the video of the issue. I’ll post a follow up with any findings once I get the car back. Thx!

 

I got my car back and spoke to the service guys. They at least experienced the no power issue once, but the battery checked out again (12.57V, 294CCA) first thing in the morning and they said they did not in good conscience want to replace it as it seemed fine...at least until trying a less expensive alternative. (Props to them for looking out for my wallet.) What they did was replace the battery in the key fob. Strange, I know, because the key seems to work fine and I can unlock the car from 20 feet away, but they said they have seen comprised key fob batteries sometimes cause very strange electrical issues in a Prius. So, without doing a deep dive into diagnostics (which could be many hours of work and $$$ and no guarantee of a fix), I went just with the key fob battery attempt.

My next step if the issue presents itself again is to use my other key fob in case the whole fob went bad, then replace the 12v battery, then go for a full diagnostic service. I'm hoping it's as simple as the key battery and all will be fine now.

I will post a follow up in a week or so to let you know if things have resolved. Thanks all!

 

Good suggestion on checking the voltage at the jump point, thanks. I'm borrowing a voltmeter from a friend today.

Update - It's definitely not the key fob or it's battery, as I still have the issue with the new fob battery, as well as with my other key fob (with the new battery in it). But I think I'm narrowing it down.

I got in the car for the second time today (it started fine this morning) just by opening the locked door (not using the key fob) and it did a half-beep, which tells me the no power issue will be present. So instead of trying to start the car, I just sat there and listened to the humming/buzzing, which started as soon as I opened the door. I'll repeat that...I did nothing with the key or the car besides simply opening the driver side door (which had been locked). I then opened the hood and listened for the location of the hum/buzzz, which seemed to be coming from below left of the fuse box/jump point area. (I'll try to pinpoint it better when it happens again.) I got back in the car and waited another minute (maybe 3 minutes total) until the hum/buzz stopped, then started the car like everything was fine. When I've interrupted the humm/buzz before with trying to start the car, I'm not able to start it.

I'll check things out with the voltmeter when it happens again and try to locate the exact source of the hum/buzz and post an update.

 

I got my hands on a friend's voltmeter but it was missing a fuse so I didn't check the car first thing this morning, but the car wouldn't start at all. The check engine light came on briefly, then dimmed, then no power at all and the buzz in the engine area went on for 20 minutes or so as I tried to locate its source, then everything stopped...totally dead. When I finally got the voltmeter going hours later, it read about 4 volts at the front jump point and also 4 volts at the battery in the trunk, so clearly the next step is to replace the battery, even though it tests perfectly sometimes.

Thanks again for everyone's comments! I'll post an update a few days after replacing the battery.

 

I took the readings on the jump point in the engine compartment on the positive terminal and the engine block. On the 12v battery itself, it was directly on both terminals.

Update - Just got back from having a new battery put in and my one start was OK. I also had called the dealership where I bought the car 2 years ago and asked if they had replaced the battery and they said no, so it very well could be the original, which would be pushing 5 years now.

I'll post in a few days whether I still have the issue given the new battery. Thanks all!