[Closed] Car alarm randomly activating.

Mine did that on cold days, particularly annoying whilst driving around town - suspect it was a wiring fault. Turns out it has an internal battery and will go off if it is unplugged from the car.

Solution was to remove the alarm and destroy with club hammer.

As part of other warrenty work, garage put a new unit in without telling me. That also went off whilst on the motorway and received the same treatment.

Could be low battery voltage.. does it get used regularly?

Loose change in one of the cubby holes!!? I kid you not, common problem on the Toyota Celica, easy to sort so worth a try.

As per Frogstop, my car will do this if the battery runs too low.

Friend of mine had this, it was caused by a water leak where the roof rails were attached.

Leaving a mobile phone in the car sets mine off every time. Also, vehicles with loud exhausts driving past can trigger it.

Spiders

Yes, spiders. When Mrs BigJohn borrows the van to move compost to the allotment but leaves it overnight I can't lock it for a couple of days until I've located all the critters.

It's a process of elimination. The alarm will be monitoring the entire vehicle such as doors/boot/bonnet, tilt sensors, voltage drop and interior motion detection.

It might be possible to disable the interior motion detectors which would rule out a false alarm there.

Years ago I had a Clifford Concept 50 alarm which was probably worth more than the car it was fitted to (But less than the amps and subwoofers) when parked in the driveway it was just outside the kitchen window where the microwave was and would go off every time the Microwave was used.

Spiders

Yes, spiders.

Have you bought any bananas recently?

Many car alarms have a back-up battery, so they can scream and shout if a thief disconnects the main car battery before breaking in. As with any other batteries, they don't hold charge after a few years. This upset my Audi alarm; it had a mournful weep until I took it to Audi. They fixed it with a new back-up battery.

If you haven't been using your car much due to lockdown, the main car battery could be getting flatter more often. I think alarms are designed to ignore this gradual voltage drop, but it might work the back-up battery much harder, reducing its life. Through lockdown I have sometimes been locking the car with the key, which doesn't set the alarm (immobiliser still works), so doesn't drain the battery so much. Insurance probably didn't cover theft from the car, though. If I'm not going to use the car for a while, I now disconnect the battery, but beware: I have heard that some modern cars are so clever/stupid, that this confuses them and can need a breakdown service or even a main dealer to coax them into working again.

Second the comment about the Toyota Celica: the transmitter/receiver for the internal sensor (think it is radar) is next to the handbrake lever, under the double cup holder in the centre console thingy. Which is perfect size and place for my favourite driving sunglasses. If I forget to take them out of it at the end of a drive, the alarm thinks there are intruders and gets frightened. It's the worst internal sensor placement I have heard of, but other cars might have something equally silly.

If your car isn't too new and too clever/stupid, removing the fuse for the alarm shouldn't cause any problems that can't be undone. I think it is a separate system to the immobiliser, but you could always try it and see. If your car should have an alarm as standard, your insurance probably won't cover you for theft from the car until you get it fixed.

Have you bought any bananas recently?

Given the OP's username I think banana's are likely

Tons of bananas - maybe that’s it. Car used daily still so doubt it’s the battery, but I’ll check the cup holders!

I had a hire car that was doing this constantly. Dose of bug spray and leave it shut for a bit cured it. The was something flying around in there being picked up by the interior sensors.

Given the OP

Barking Spiders

Is it an older VAG car? They used to leak water in via the plenum chamber and flood the ECUs, which set off the alarm.

Barking Spiders

I didn’t think fogginess would set it off all by itself. Could be a spider or something I guess, we do use it for tip runs and so on. It’s a Toyota Verso.

Had this on my Trafic, turned out to be the drivers door switch, I stuck a square of plastic on the door where the switch touches it to put more pressure on the switch with the door shut, had every intention of replacing the switch... it sorted it and its still there a year later.

Mine does it very occasionally, goes off, then keeps going off randomly. Usually a bug or some such causing it. Happens maybe once a year.

Another VW group random alarm fault that generates the OPs problem is the washer hose splitting on the way around the boot hinge and soaking the boot electrical connector directly beneath it.

When mine went I was told reasonably common on Golf and Passat estate and others on same platforms.

If it's a vw group car worth a check for a wet headlining by the boot hinge on drivers side.

I managed to get mine to stop until it could get fixed by drying the area with a hair dryer and then leaving a heater in the boot for a couple of hours to drive the moisture out.

Some cars with volumetric alarms can have issues if vents are left open

Had this with a moth that managed to sneak in and hide, took a while to get him out.

I’ll give it a good clean out then!

+1 for remote alarm siren battery. Our Volvo has a remote sounder hidden behind the inner wheel arch cover. The onboard battery dies after quite a few years and it randomly trigger unless you pull fuse #11 or replace the battery/buy a new replacement sounder.

Got switches on the doors/bonnet? Duff switch in the way out + wind causing a bit of movement in a door/bonnet = alarm. Replace switch, life is quiet again. Had it twice, two different makes, same problem. Bonnet switch. Maybe the heat in the engine compartment is unkind.