2005 Prius wont start and horn sounds continuously

welcome! reverse polarity can be hazardous to your health. take it to the dealer and pray that you don't need a new inverter. if you do, look around for someone who can install a used one from a junk yard or off of ebay. all the best!

 

welcome! reverse polarity can be hazardous to your health. take it to the dealer and pray that you don't need a new inverter. if you do, look around for someone who can install a used one from a junk yard or off of ebay. all the best!

 

Yep. Reversed polarity can sometimes (often) result in a needing to replace a multi-kilobuck inverter.

OP also bought the wrong Optima. Optima DS46B24R Optima battery direct fit replacement for Prius 2004 + & Plug-in Prius with Instructions is direct fit. Elearnid used to sell Toyota Prius 12 Volt Auxiliary Battery with install kit for 2004 + including Plug in before the direct fit version existed (in the US?).

Priuses w/o SKS have a smaller 12 volt than ones with.

 

How is the new battery you put in? Is it fully charged? If not, give it a good charge before trying anything else.

 

^^^
That Optima battery (as long as the terminals are on the correct sides) is likely usable in a Prius w/install kit (new + terminal and new - cable).

 

I doubt that it's anything to do with your reversing the polarity. I just had the exact same problem happen to me, but the issue started completely on its own without touching the car - at 11:30 at night. Neighbors must have loved it. Unlike you, I could not figure out where the heck the horn fuse was. Anyway, the only thing that I thought had happened was that the 12V battery had died. The interior lights had been getting more and more dim, power windows were slow to roll up, sometimes I'd have to jump start the car if it sat for a week or so. So I figured it was time to change the battery. I bought a new one, put it in, and as I plugged in the larger white connector above the 12V battery, the horn again started going off and all the other same symptoms. I'm stumped. Would sure love to know if you've resolved this problem or not, and what it was. Thanks!

 

Well, after an hour or two of research on the web, removing the two front horns, and starting to think this was going to be a large bill from the dealership, I put enough clues together and figured it out. Simple. Page 46 of the manual: "If the auxiliary battery becomes discharged due to the vehicle being unused for a long time, etc., when the battery is recharged or replaced, the system will sound the alarm. If this happens, immediately stop the alarm."

Duh. The last thing you'd think to do - simply turn off the alarm by unlocking the doors with the FOB. I guess what originally happened is that my prior battery weakened to the point where it would electrically "hover" right at the voltage threshold where the system thought the auxiliary battery was being reconnected over and over, and so the alarm would go off for no apparent reason. Obviously, replacing the auxiliary battery would cause the same problem one more time, but now that I've simply turned off the alarm and have a fresh battery, it shouldn't happen again for years. I've also read this can be caused by a stuck panic button on the FOB, or a bad FOB sending a false panic signal, or electrical interference from external sources.

 

One more postscript: When I started the car after replacing the battery, the hybrid system warning light came on. I read on another thread that this happens any time you mess with the wiring. Power the car on and off three times, and the problem goes away.

 

I've had the same problem on my 2002 Prius.
Only after finding this thread did I finally get around it. Just to reconfirm, when turning on my jumper battery (that's connected to my 12V battery) I repeatedly pressed the unlock button on my remote and finally the horn no longer sounded.

Strange, as during previous jumps, I didn't run into the horn issue but it seemed to suddenly happen every time. Thanks everybody!

 

This string is common among problems that CAN be experienced when changing the 12 volt battery in the Prius.

1. Understand that the 12 volt battery OEM battery comes in two flavors.

1A. A SMALLER battery for the base model car.
1B. A LARGER battery for the NON base model's.

The OPTIMA batteries being sold today also come in different flavors. Be aware of this fact. The part #'s above spell out which is which. Take your time and research this fact BEFORE you order online. As stated these batteries are difficult to return due to HAZMAT rules.

FWIW, It is easiest to simply deal with a local Toyota Parts Dept. They stock all necessary parts to swap the battery with no problems.

If one is working with a BASE model Prius and gets the bigger battery that just won't fit correctly (sides rubbing, strap clamp on top not fitting) the correct fix is to CHANGE the TRAY that the battery sits in. Toyota DOES stock the tray. Any good parts guy at Toyota will know this fact if they are PRIUS AWARE.

Best solution IMO. Buy OEM and get the bigger battery. Swap the tray on the base model units. By doing this you get the larger battery with the additional amp/hrs. to help it possibly have more strength and longevity.

Also FULLY charge ANY battery you put in BEFORE you install it.

And yes, the security system will sound off if the battery has been dead for a while. And yes, some times it takes more than one push of the power button to get everything back online.

Just be patient, use the right parts and your 12 volt battery swaps will be non eventful.

Also, under NO circumstances should you ever reverse the polarity on these Prius cars. It only takes a split second to do a LOT of damage. The fuses are not cheap. You can damage more than just fuses also. Plenty of posts here on PC about horror stories of bad jumps.

Bottom line, if you need a jump - YOU need a NEW battery NOW. Don't keep jumping the car. Each jump is a risk. Sooner or later that live round will come up in the cylinder.

Peace Everyone,
Ron (dorunron)

PS See my link below in my signature file on how to change the 12 volt battery. Fully documented for Gen II Prius. Gen III is easier. NO plastic vents for traction battery to move...