What is an immobiliser?

Last updated by mustard.co.uk on November 8th, 2022 and verified by The Parkers Team

An immobiliser can protect your car from thieves as well as help lower the cost of insurance,  but what is an immobiliser, how does it work and how can you tell if your car already has one?

How does an immobiliser work?

A car immobiliser is a security device that stops your car from starting if the wrong key is used. You can get electronic immobilisers and physical immobilisers which include devices like steering wheel locks.

Most modern cars have electronic engine immobilisers. For your car to start, your key or key fob sends a code to a part of the car known as the electronic control unit (ECU). If the code is correct then the car will start by activating the ignition, fuel and starter motor systems.

If the code is wrong or someone tries to hotwire the car, electronic immobilisers will stop the car from starting by preventing the ignition, fuel and starter motor systems from working together. Immobilisers will vary by manufacturer but the basic principle behind them is the same.

Related: How to keep your car secure

What is a factory fitted car immobiliser?

Factory fitted car immobilisers are installed during the manufacturing process. They’re considered more desirable than ones added on afterwards because it’s assumed a device installed at manufacture is more likely to be robust and fit for purpose.


car Immobiliser dashboard icon

Does my car have an immobiliser?

Since 1998, immobilisers have been fitted as standard in all cars. So, if your car is less than 20 years old, there’s a good chance it will have a factory fitted immobiliser — unless a previous owner has taken it out or disabled it.

If your car was made before 1998, it may still have an immobiliser as some manufacturers did fit them before 1998. A previous owner might also have put one in. If you’re not sure, check your car handbook or ask a mechanic at your service or MOT.

Related: The best van security locks on the market

How does an immobiliser affect my car insurance?

A car  immobiliser can cut the cost of your insurance because it should make your car harder to steal.

If your car doesn’t have an immobiliser or you’re thinking of buying one, we’d recommend buying an approved Thatcham security device. These are the preferred choice for many insurers and are likely to result in the biggest savings.


What is a Thatcham device?

Thatcham Research is a not-for-profit group who work with manufacturers, trade bodies and government agencies to promote car security and safety standards.

Security devices and car immobilisers approved by Thatcham have been tested to their rigorous standards which is why they’re often recommended and preferred by insurers. 

There are seven Thatcham categories (previously eight) which show the different types of car immobiliser or security device available. It’s worth knowing that these groups simply highlight how the devices function rather than act as a grading system. 

Devices that aren’t Thatcham approved fall outside of these seven groups and are known as ‘Q class’ devices.

Here are the seven Thatcham categories in detail.

Thatcham Category 1: Electronic alarm and immobiliser These systems have an electronic alarm and immobiliser. They include perimeter, ignition, glass breaking and tilt sensors. The immobiliser must also isolate at least one of your car’s control units. The system is passively set which means you don’t need to do anything for it to work.
Thatcham Category 2: Electronic immobiliser The same as category 1 but without an electronic alarm.
Thatcham Category 2.1: Electronic alarm upgrade This is where a category 2 system has been upgraded to category 1 because an electronic alarm has been added.
Thatcham Category 3: Mechanical immobiliser These are physical pieces of equipment rather than electronic systems. They include devices like a Thatcham steering wheel lock.
Thatcham Category 4: Wheel locks These are devices that lock your wheels and include products like the wheel nuts used for alloys. These are considered a theft deterrent because you need a special key to unlock them.
Thatcham Category S5: Post theft tracking Previously just known as category 5, group S5 can track and locate your car when it’s been stolen using GPS. Not only that, the car can also be immobilised remotely. 
Thatcham Category S7: Stolen vehicle location Cars have a tracking device but they can’t be immobilised remotely.

Does my car have a Thatcham alarm or immobiliser?

To find out if your car’s been fitted with a Thatcham immobiliser,  check the manual or ask the manufacturer. 

Related: How is car insurance calculated?

I’m having car immobiliser problems, what should I do?

On rare occasions, your immobiliser might not work and you could find yourself unable to start your own car. This could be down to a dead battery in your key fob, in which case, a simple battery change should solve the problem.

If you can get into your car, but the engine won’t start, take the key out of the ignition, leave it for a few minutes and try again. Alternatively, you could try leaving the key in the ignition to see if this reactivates the system.

Some key fobs also come with manual keys (you usually have to slide the cover off the fob). If the fob fails to work, then using the physical key should work.

If you’re still having problems, it’s best to call a local garage or the dealership you got your car from. They should be able to arrange a replacement key fob or find out if there’s a mechanical or electronic fault.

car keys in ignition

How does an immobiliser effect my car insurance?

The more secure your car is, the less likely it is to be stolen, so having an immobiliser can cut the cost of your car insurance.

If your car doesn’t have an immobiliser or you’re thinking of buying one, we recommend buying an approved Thatcham security device as these are the preferred choice for many insurers and are likely to result in the biggest premium savings.

If you want to find other ways to lower the cost of car cover, then read our guide on how to save money on your premium.

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