Alarm Going Off For No Reason

[Dave R.]

Had a frustrating experience at the weekend with the alarm going off for no apparent reason (Gen 3 T-Spirit). As far as I'm aware, all the windows were closed and there was nothing in the car likely to be moving to set the motion sensor off. The only thing I can think is that it was a hot, sunny day.

The alarm triggered.... I went and unlocked the car to cancel the alarm... started the engine to check that all was OK.... stopped the engine and locked the car again. A few minutes later the alarm went off again... again, clicking the remote stopped the alarm. This repeated 2 or 3 times. In the end I left the car unlocked for a while (not a problem as it was sat on the drive). When I locked it up again a bit later, all was OK and (touch wood) it hasn't gone off again.

One point... the red "alarm/lock" symbol was flashing on the dash... but I can't remember what it should do normally.... I know it will flash slowly when the car is locked and the alarm set... but when does it go out? Is there a way to "clear" alarms? The manual doesn't appear to be very informative.

[Jan&tone]

Go talk to Mr T... Unless there is movement in the vehicle, or someone was breaking in, there is no reason for the alarm to go off.... The 12v Battery can sometimes cause odd happenings but shouldn't be in rhat state yet!

As to the alarm light on the dash, I believe it is static for a few seconds then flashes till you unlock the car next time you use it...

[steverally]

Hi, seemed to remember a similar problem being raised on forum a few weeks ago, think it was resolved by replacing 12V Battery.

Have added link - hope you can access, otherwise scroll through past posts for 'alarming issues'.

http://www.toyotaownersclub.com/forums/index.php?showtopic=122789&pid=1103957&st=0entry1103957

[Dave R.]

Hmmm sounds like I'm going to have to keep an eye on the 12V Battery. Thanks for the pointers.

Just by coincidence, the "power sounder" (siren) on my wife's Tigra kept going of a few months ago - that ended up costing about £200 & resulted in removing the siren from the car (siren sounds if the alarm is being tampered with - another potential Battery problem!... of course, if it goes faulty, you can't stop it as it has a mind of its own!)

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[Seamaster73]

In some cars an unexpected voltage drop can cause the alarm to sound (the alarm system interprets this as a door being opened/courtesy light coming on). I don't know whether or not this applies to the Prius's system, but it would be worth ruling out.

[Jamesbelfast]

Car alarms work on the voltage drop principle so even a slight voltage drop in the car electrics can cause them to activate.

Are there any blue-tooth or mobile phone facilities fitted?:)

[Norfolk n Chance]

A common issue on the Corolla's was the bonnet lifting slightly when left in the hot sun.... this would cause the bonnet sensor on the alarm to lose contact with the bonnet ,only by 1mm or so, but enough to set the alarm off.

[Grumpy Cabbie]

I'd be tempted to double or triple check that the rear hatch is fully closed. Perhaps you could turn off the light inside the boot and see if that helps?

Worth a go if nothing else.

[Dave R.]

Well, thanks for all the responses. It's been OK ever since (touch wood).... I'll keep the forum posted if it goes off again!

[Sayling]

I had an incident last week with my 08 Prius where the alarm went off several times. I had my iPod touch connected to the AUX input (but not playing and in sleep). When I disconnected the iPod, the alarm stopped going off.

Sayling

[1rickster]

I have had similar issues with my 2005 Prius.

The first time it turned out to be one of those bluetooth hands free telephone things that clip on to the sun visor. Removed this and hey presto. It never went off again! Until..... The other day it was doing it again. This time it was down to a wifi enabled tablet that my son had left in the seat pocket. Once again, removed this and had no issues.

I guess the signals that these devices give out interfere with the alarm system.

[Grumpy Cabbie]

I would seriously suggest you check the health of your 12v Battery - especially if it's the original.

I have never heard of alarm issues on a Prius caused by wifi or bluetooth, but I have heard probably hundreds of reports of Prius alarms going off when the 12v is failing.

They generally last 5 years or 80k miles. I'd guess you're on your second 12v if you have a 2005. Don't rely on the car telling you it's ok and don't rely on most dealers telling you either as generally they rely on the car. There are tens of threads on here about failing 12 volts with details of how to check.

[PeteB]

very true, but there have also been many reports of alarm problems ceasing when metal rimmed glasses or other metallic objects are removed from the overhead glasses storage compartment.

[Grumpy Cabbie]

One or two, and a few more issues with the rear hatch not fully closing. But a 10 year old Prius with issues reflective of a 12v failure would suggest that it is at least checked first. Then we can look at potential water incress into the interior light/sunglasses compartment - assuming they person comes back :)

[Merlock]

I'm having this issue and haven't found the root yet. Will try all the suggestions and report back.

[kithmo]

SWMBO has this problem with her 06 and we never resolved it either. We replaced the 12v and it seemed to be ok for about a month then it started again. This makes me think it's something to do with a voltage drop somewhere. There are no lights on, we even turned the interior lights completely off. She now disables the internal sensor every time she turns the car off now and it doesn't go off.

[Merlock]

That's my situation at the moment, I have to press that button every time I get out. And had a new Battery fitted by Toyota less than a year ago.

[PeteB]

13 hours ago, kithmo said:

SWMBO has this problem with her 06 and we never resolved it either. We replaced the 12v and it seemed to be ok for about a month then it started again. This makes me think it's something to do with a voltage drop somewhere. There are no lights on, we even turned the interior lights completely off. She now disables the internal sensor every time she turns the car off now and it doesn't go off.

This makes me think it might not be the 12V (but can't be certain, because a 12V voltage drop does cause all sorts of unusual symptoms on a Toyota Hybrid).

The reason I wonder is that the cut out button disables the ultrasonic internals sensors that would be set off by pets, children, OAPs etc. left in the car, while still leave the voltage drop part working.  This is easily checked by single locking (not deadlocking with 2 presses of the button) the car with a window open, then reaching in, manually unlocking and opening a door.

[barrycoll]

As mention in my similar post of a few months ago, my G4 2017 alarm was sounding off regularly.

checked all the closures etc, and then put the “newish” 12 V on the  CTEK Smart charger for 48 hours, as it was far lower on the units readout than you would expect.

After the 7th green light came on from the CTEK, problem solved.....but guaranteed to be the 12 v, despite its age