Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

2004 Prius Won't Start - Bad Hybrid Battery Pack?

Hello. I’m new to the forum and looking for some advice about my 2004 Prius. As of today, the car won’t start. I put in the “key” and press the power button, but nothing happens. Also, no dash lights come on, the power locks don’t function and the dome light doesn’t come on, so the car does not appear to be getting power.

I pulled out the 12V battery and brought it to an auto supply store. They tested it and told me it was still good, so I’ve ruled that out. Because of the age of the car, I’m worried that the hybrid battery pack may be bad. This didn’t just happen out of the blue. Here is how things progressed over time…

-About 8 weeks ago while I was driving, a red exclamation point warning light in a triangle appeared on the dash display. Being descended from a proud line of Vermont rednecks, I chose to ignore the light and continue driving. Two days later, the light went off.

-Flash forward six weeks. When I first tried to start the car it was totally dead, as described above. However, after two or three tries, it started.

-Over the next two weeks, the not starting issue continued intermittently, becoming more and more frequent, which leads us back to today and the beginning of my post.

Any advice is welcome. The other thought I had was could this be a bad starter, but given the push button ignition I have no idea if the symptoms would be the same as an old fashioned turn key kind of car. The progression does remind me of what I’ve experienced with that in the past (progressively more difficult starting until finally the car won’t start at all).

Thanks for the advice.

I don’t know anything about those battery packs, but would think there are instructions in the manual that describe what to look for and maybe even how to test those batteries.

The only other advise I have is, don’t ignore instrument warning lights

Ha! Yeah, ignoring those lights does not tend to be the best idea, turns out. Thanks for your suggestion.

If that 12V battery is still the original, I would replace it ASAP

By the way, the easiest place to buy them is at the dealer, as it’s a strange size used only in the Prius

If this were not a hybrid, I’d say check the (12v) battery connections, + and –, both ends. Disconnect, clean and reconnect. Since it goes totally dead, it’s not the starter.

But I have no idea how the hybrid battery interacts with this…

But db is correct, if it’s the original battery, replace it. But check the cables first.

If the 12V battery is weak it causes major problems. How old is the 12V battery? It’d be worth checking it carefully, even replacing it to potentially save big $$. But a 13 year old Prius could need a new hybrid battery. A Toyota dealer can check it out.

Thanks all for the ideas. I replaced the 12V battery 1.5 years ago, so doesn’t seem likely that is bad (especially given that the one store at least told me it was ok).

As far as I could tell, the terminals are clean and everything is well connected, but worth a second look at all of that to prevent spending bug $$ I agree.

A reply on another forum led me to believe that the 12V battery is responsible for starting up the dash lights, computer, etc… and sending a signal to start the car, at which point the hybrid battery kicks in and the car actually starts.

That would still tend to point to a failure of the 12V battery or something pretty early in the startup system prior to the hybrid battery. Would the computer ever go on one of these? I wonder if that could cause something like this?

If anyone cares to opine, I’m listening. Thanks!

Yes, the 12V battery controls the electronics, the hybrid battery starts the engine. So if either one is bad, the engine won’t start.

Get a voltmeter and check the 12V battery in the car. You’re flying blind here…


Based on your question, you don’t have a detailed knowledge of how cars function. If that is true, take it to a qualified independent shop for help. There are a lot of Priuses in Vermont, so there are mechanics.

You wouldn’t do your own appendectomy or cataract surgery would you?

The manual for your car says this:

So one of the system warning indicators should have focused in on the offending system:

Here’s what I find interesting- note what should be done in most cases:

Having that tidbit of information would be helpful in understanding if it’s contributing to the problem you have now.

If the 12V battery supplies all of the electronics for dash etc and they are dead but the battery tests OK, then I would be looking for some type of master fuse to see if it’s blown…

Maybe the Neutral Safety Switch is out of order. Or possibly the Start Button is likewise.
If you have a “chip” key that could be a cause also. If the key is dirty, clean it with a toothbrush.