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What is an immobiliser and does it affect my car insurance?

Do you have a car immobiliser? We explain how to find out and why this clever piece of equipment could save you money on your car insurance.

Do you have a car immobiliser? We explain how to find out and why this clever piece of equipment could save you money on your car insurance.

Written by
Rory Reid
Car and technology expert
Last Updated
19 JULY 2023
6 min read
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What is an immobiliser?

An immobiliser is part of your car’s security system and will stop your vehicle from moving if someone’s trying to steal it.

There are two kinds of immobiliser: 

  • Mechanical immobilisers, such as steering wheel locks and gear clamps.
  • Built-in electrical immobilisers, which we’ll be focusing on in this article. 

How does a car immobiliser work?

Electrical immobilisers stop your car from starting if someone tries to hotwire it or uses the wrong key or key fob. The precise workings are usually kept secret to make it harder for thieves to disable them.

Basically, if the code sent from the key doesn’t match that of the car’s electronic control unit (ECU), the immobiliser disables two or three key components of your car. These include the fuel system, start motor and ignition. It can also trigger the car alarm. 

Most vehicle immobilisers today work passively, which means you don’t have to set them manually. This means there’s no risk of you forgetting to turn them on. However, it does make it harder to tell if you have an immobiliser installed, which might be necessary if you’re ever trying to diagnose a fault that involves the car not starting.

Does an immobiliser or alarm affect my car insurance?

Because they can reduce the risk of your car being stolen, having an immobiliser and car alarm could also cut the cost of your car insurance. However, this may depend on your insurance provider.

Your car may already be fitted with an immobiliser. If it’s not, you can have one fitted.

What is a factory-fitted immobiliser?

Factory-fitted immobilisers are installed when the car is made. They’re often considered to be the most effective, as they’re likely to have been designed for your car and are fitted properly.

Does my car have an immobiliser?

Factory-fitted immobilisers have been compulsory for new cars in England and Wales since 1 October 1998. If your car was manufactured after that date, it’s likely to have one, unless it’s been imported or a previous owner removed it.

The simplest way to find out if you have a factory-fitted immobiliser is to check your car manual or get in touch directly with the manufacturer. Most car manufacturer’s websites will provide their contact details.

If you still can’t find an answer, a mechanic will be able to tell you.

What is a Thatcham device?

You might also be asked about ‘Thatcham-approved security systems’ when you’re looking for car insurance. Thatcham Research is an organisation that certifies alarm systems, rates the security of new vehicles and defines car insurance groups. Its certifications for alarms and immobilisers have become an industry benchmark.

You can search for factory-fitted Thatcham approved alarms on the Thatcham website. This also includes mechanical immobilisers and tracking systems.

What’s the difference between Thatcham Category 1 and Category 2?

There are seven categories (or classes) of Thatcham certification. The lower the number, the more security that device offers. Insurance providers usually ask for Categories 1 and 2.

  • Category 1 includes an electronic alarm and immobiliser. It’s the most secure system available.
  • Category 2 includes an electronic immobiliser without an alarm. 

There’s also a Category 2-1, which involves installing a Thatcham assessed 2-1 alarm when you have a Thatcham Category 2 immobiliser already in place. This gives your car Category 1 status.

Does my car have a Thatcham-approved alarm?

Not all models have a Thatcham-approved alarm fitted as standard. To find out, check your car manual, or get in touch with the manufacturer or the retailer you bought the car from.  

If you want to install an alarm, always choose an approved Thatcham device. Not only are they likely to be more effective, but they’re another way to potentially save on your car insurance policy.

Are immobilisers effective?

Factory-fitted and Thatcham immobilisers are very good, but electrical devices can never be 100% foolproof. Sophisticated criminals tend to rise to the challenge of new technology, and keyless car theft is a possibility through what’s known as a relay attack. This uses electronics to extend your key’s signal making the car’s technology think the key is nearby.

Generally speaking, issues are rare and immobilisers are considered effective anti-theft devices, particularly when you combine them with other sensible precautions.

These additional security measures could include: 

  • Parking your car in a garage or a secure, well-lit location, ideally with CCTV.
  • Not leaving valuables or cash visible inside your car.
  • Making sure your car is properly locked when you leave it.
  • Storing car keys away from windows and front doors where they might be in plain sight.
  • Keeping your key fob in a faraday pouch at home to prevent thieves intercepting the signal.
  • Deactivating your key fob at night – check your car manual to find out if this is possible for your particular model.
  • Testing your car alarm regularly.
  • Investing in a steering lock. These prevent your steering wheel from moving unless you remove them with a physical key.
  • Adding a GPS tracking device. These let you locate your vehicle if it’s stolen.

There’s more about securing your car in our article on how to prevent keyless car theft.

Frequently asked questions

Does an immobiliser count as a modification?

Any changes to your car that mean it no longer matches the manufacturer’s original specifications are known as modifications.

Factory-fitted immobilisers are part of this original spec. If you fit an immobiliser or buy a car with an after-market immobiliser on board, it might count as a modification depending on your insurance provider.

If this is the case, you’ll need to think about modified car insurance. It shouldn’t cost you more though – the good news is that by making your car more secure, you’re likely to save money on your premiums.

Can I fit an immobiliser into an older car and will it impact my insurance?

If your car hasn’t already got an immobiliser, you can get one installed. But to stand the best chance of reducing the cost of your car insurance, you’ll need to make sure it’s Thatcham-approved and fitted correctly.

How can I check my immobiliser works?

Putting your key in the ignition sends a code from the key to your car’s electronic control unit (ECU), which has a matching code. This will start your car. If you try to use the wrong key or don’t have the key at all, the immobiliser will stop the engine running. It may also trigger the car alarm.

How do you fix an immobiliser problem?

If you’re having problems with locking/unlocking, your engine starting or your keys aren’t working, you may just need to change the battery in your key fob.

However, if problems persist, get in touch with the garage or dealership that sold you the car. There could be more serious electrical issues that are causing the problem. If so, you’ll need to speak to a mechanic. 

Are there other security devices that work alongside an immobiliser?

Most cars have an audible car alarm included as standard. But while this may deter a car thief, it often gets ignored due to the high number of false alarms and the difficulty telling which car’s alarm is going off.

Silent alarm systems are another option. There’s no noise, but you’ll get an alert if someone tries to break into your car. Never try to intervene if this happens. Call the police and stay away from any potential danger.

You might also want to install GPS tracking. It won’t stop your car being stolen, but by showing where your vehicle is in real time, it makes it easier for the police to find it.

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Rory Reid - car and technology expert

Rory Reid is a car and technology expert. He serves as the main presenter on Auto Trader’s YouTube channel and was previously a host on BBC Top Gear and its sister show Extra Gear. He is also a presenter on Fifth Gear. Previously, he hosted Sky TV’s Gadget Geeks, CNET’s Car Tech channel, BBC Radio 5 Live’s Saturday Edition and on the YouTube channel Fast, Furious & Funny.

Learn more about Rory

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