Anti Theft Alarm / Horn won't stop

Have you ever replaced your 12v battery? If not, you should get it checked. Very possibly it's on its last legs. Since the 12v battery powers all of the computers, odd things happen when it starts to go bad.

Also, you should check your owner's manual as to the location of the battery. It's not under the hood, and it's quite possible to disconnect it.

 

It seems this is the forum's response to 80% of the problems posted. The analog to Microsoft's "reboot your computer" response.

I can see how an intermittent connection would cause the alarm to sound, but not a low battery voltage...

 

maestro8,

It is reasonable for you to question the preliminary diagnosis of a weak
12V battery.

I dont believe that anyone here --with the possible exception of Patric
Wong -- "sees" the connection between a low 12V battery and the
numerous strange happenings that have been reported over the years.
The goings on in and between the numerous computers and ECU has
remained pretty much a mystery since the Gen I.

There are some easily explainable non-desireable conditions that come
from a low 12V battery. These include:
* reduced MPGs
* ICE running at stops even after the car is warmed up

However, and more to the point, IIRC, all the following strange
happenings were resolved by getting a new 12V battery. As the relationship
is direct and one-to-one, I think they can be considered sympyoms:
When the car is turned off:
* the headlights come on and blink
* the windshield wipers come on
* the horn blows
* the car will not start
* and maybe the windows cycling up and down

When the car is running:
* the car cannot be turned off

I sure don't "see" a connection. But that's the history.

Let's wait and see what the OP says about the condition of the "usual
suspect" in cases of electrical weirdness.

 

If it's not the 12v battery...

You might have a bad door switch. If the car "thinks" the door is being opened while the alarm is armed, then it will set off the alarm. There may be alarm related swtiches on the rear hatch and hood too. I think most cars use the plunger switch on the door jam to tell if the door is open or closed, however some cars rely on microswitches inside the actual lock mechanism (VW, for one, does this). I'm not sure about the Prius.

My 2002 VW had a bad switch on the hood. The alarm would go off randomly. Luckily with free software (VAG-COM) and an OBD-II to usb cable I was able to determine the last switch that set the alarm off. I simply unplugged the switch at the hood latch and it solved the problem. As far as I know, there's no way to tell which switch is setting off the alarm in the Prius.

Does anyone know if there is a way to lock the Prius without arming the alarm? Could you accomplish this by manually locking the doors before closing them (and not using the key fob)? I'd go out and experiment on my car but it's getting late and I don't want to disturbe my neighbors.

Scott

 

Check the panic button on each fob for a sticking button.

Then, let's talk about how to disconnect the battery. There are at least two ways:

1) Open the rear hatch, and find the 12V battery located under the right-rear hatch floor corner. Remove the negative battery cable where it bolts to the body.

2) Find the main relay/fuse box in the engine compartment, next to the inverter. Open the box cover by pressing hard on the catch at the front of the cover, then pivoting the cover up. Find the red plastic cover over the dedicated positive jumpstart terminal and open the cover. You will see a threaded stud with a nut on it. I believe you will need a 10 mm socket to remove the nut. Then you can pull off a wire that is attached to the stud, which disconnects the positive side of the battery from the vehicle's electrical system.

Once you know how to disconnect the battery so that a honking horn does not impair your and your neighbors' sleep, the next question is what is causing this, if you have ruled out a fob problem. It might be a low 12V battery, it might be a faulty door or hatch switch, an intermittent wiring harness connection, the body ECU might have a problem, etc. If your car has an aftermarket alarm system installed, that needs to be checked out.

You can check 12V battery status by turning on the headlights to see how bright they appear. If dim, the battery needs to be replaced.

If you have a digital multimeter, use that to determine battery voltage after the battery has rested overnight (which helps to dissipate any "surface charge" which inflates the battery voltage reading.) A fully charged AGM battery will measure 12.9V; a battery in OK condition will measure 12.5V or better.

If you can't easily find a problem then I suggest you seek professional help for your car. Good luck.

 

Hey, so I've been having the same awful issue. Last nigh it went off 20 times at 3 am.
Tonight I tried leaving my hood open, because when I took it to Toyota they thought it was my hood latch and since I left it open, it hasn't gone off yet.
Not sure if it's a similar issue, but I hope if it's too crazy like mine this could help.
Good luck

 

I had similar problems with 2006 Prius. Put a new battery in the fob. Didn't help. Then the panic (red) button fell off while I was diddling with the fob. That seemed to solve the problem. I appreciate the remarks that the horn blowing, light flashing was not likely the result of a a low 12V battery, or the hybrid battery. I only have 138K miles on mine and was not looking foreward to the expense of of replacing the batteries, which I am sure the local dealer would have been happt to do!