After your car alarm has gone off in the middle of the night for the third time that week, no doubt waking you and your neighbours, it moves beyond irritating and into embarrassing. And in addition to disturbing those around you, a car alarm that's on the fritz could invalidate your car insurance. As if that wasn't bad enough, it could even result in a 'statutory nuisance' notice from your local council. 

So, why does your car alarm keep going off? To help you get to the bottom of things, we'll take a closer look at the most common causes, as well as how to remedy the issues and keep those noisy nights to a minimum.

Dead battery in key fob

Your key fob contains a short-range radio transmitter that sends a distinct signal to a receiver unit inside your car. When it's in proper working order, it locks or unlocks doors and jumpstarts your car ignition at the press of a button. But when it's faulty, it's unable to send the signal to the receiver unit inside your vehicle, which causes your car alarm to go off for no reason.

How can I fix it?

In most cases, a new battery is enough to sort the problem. But there's a chance it may need to be reset too. Try taking it apart to see if there are any broken contacts or wonky buttons. If everything seems fine but your alarm is still going off, then you might need to reprogram the controller which you can do on your own or with the help of a professional.

Defective bonnet latch sensor

As well as your key fob, other sensors in your car might cause your car alarm to go off. Take your bonnet sensor, for instance. A fully functioning bonnet sensor picks up when your bonnet latch is open, closes the electrical switch and illuminates the dash with a warning light. When the bonnet sensor is defective, the car alarm goes off instead.

How can I fix it?

Since the sensor sits in the engine bay above the headlight frame, it tends to collect dirt, debris, and engine grease with ease, which causes it to trigger these false alarms. Giving the bonnet latch and sensor a clean can stop your alarm going off and protect your vehicle from theft and damage too.

A brake cleaner and cleaning brush will usually do the trick. If the alarm is still going off, then your bonnet may be damaged, or it may have been tampered with. If so, replace it as soon as you can.

Poor bonnet latch connection

If your alarm is still going off after cleaning or replacing the bonnet latch sensor, then it could be an issue with the bonnet latch connection itself. Although it rarely happens, when it does you should take note. A faulty bonnet latch can also affect your car's control unit and lead to incorrect commands and reading, such as turning the airbag light on – even when there isn't a problem with the airbag function.

How can I fix it?

If there are no signs of damage, then disconnecting and reconnecting the bonnet latch connection usually sorts it. If not, then look for cable fraying or binding, setting of rubber stops on the bonnet, the condition of the pull handle assembly, or any wear or damage to the actual latch. Also, check to see if any of the levers or springs are worn out or bent. If so, you should have it looked at by a professional.

Faulty door lock sensor

The design of a car's door lock sensor means that rain can seep through the door lock actuator connector, which causes the car alarm to go off. Its internal wirings can also wear out and fail too.

How can I fix it?

Sealing the connector with protective grease and identifying the source of the leak fixes the issue. Keep an ear out for any buzzing or humming too, as this can help narrow down whether the door lock actuator is working or not.

In the case of faulty wiring, a scan tool can help determine which vehicle door has caused your vehicle alarm to keep going off the most. Whichever door it is, you should inspect the door lock connector and actuator to see if any of these parts require replacement.

Low car battery

If your car alarm goes off in the middle of the night and your car battery is dead the following morning, then the car battery is likely the main culprit. One of the main functions of a car alarm is to warn the driver about low battery levels. That's why turning the engine on with a dying battery will instantly set your car alarm off.

How can I fix it?

Using a voltmeter, check to see if its running voltage reads less than 12.6V (or whatever your owner's manual specifies). If so, you'll have to jumpstart, recharge or replace the battery.

Poorly installed alarm system

Factory alarm systems only provide a basic level of security and anti-theft protection, which is why many vehicle owners opt for an aftermarket car security solution. Unfortunately, that leads to improper installation, especially when motorists take the DIY route. If your car alarm keeps going off at night, then it could be because of the shoddy installation job.

How can you fix it?

If you tried installing it yourself, then double-check the instructions in the installation manual to make sure everything you did followed everything properly. If a professional installed it, then explain what's going on to them. A proper reinstallation of the alarm system will stop it from blaring away at night.

A word on statutory nuisances

In certain cases, a malfunctioning car alarm might be considered a 'statutory nuisance' by your local authority, especially if you live in a built-up area. When this happens, you might be issued with a warning notice, or even have your vehicle impounded, with any associated costs passed on to you.

If your factory-issue alarm has started to malfunction, then it's important to check to see if your warranty covers it. It's also worth investigating whether after-market alarms invalidate your warranty at all.

A malfunctioning alarm could also cause problems if you need to make a claim with your insurer for a break in or vandalism.

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