A guide to car immobilisers

What is a car immobiliser and how does it work?

A car immobiliser is an electronic security device that stops your engine starting unless you use the correct key. If you don't have the right key, your engine won't fire up.

There's a small computer that sits in your car's ignition system and the engine control unit. This pairs with a chip in your key called a transponder. If the chip in they key and the ignition fit, your car starts.

If they don't match, the system blocks the critical circuits. Usually the fuel pump, ignition coil, or starter motor. So, even if a thief tries to hotwire a car, it won't work.

Since 1998, immobilisers have been mandatory on all new cars sold in the UK. Factory fitted immobilisers are usually fitted bt the car manufacturer before it even leaves the factory. They're integrated into the car's ECU and ignition system - so they're extremely hard to remove.

Aftermarket immobilisers are fitted after the car is build by a specialist fitter. These tend to be more common in cars built before 1998 for owners who want extra protection. If you're getting an aftermarket immobiliser, then it's a good idea to make sure it's Thatcham approved.

What are the Thatcham security categories?

Thatcham Research is an organisation that regulates and rates security and safety standards for the UK motor insurance industry. They sort immobilisers into different categories depending on the product:

Thatcham Category 1

This is the highest standard for immobilisers, and combines both an alarm and an immobiliser. Systems in this category are probably the most complex on the market. This type of system is the gold-standard of protection, and it doesn't just stop criminals from stealing your vehicle. Thatcham Category 1 alarms have movement sensors, which trigger at the slightest movement when your car is locked.

It also has perimeter and ignition detection, as well as a siren that has its own battery. This activates if your car is broken into.

Thatcham Category 2

This category is just for immobilisers, there aren't any alarms in this category.

Immobilisers in this category must have the same technical standards as the immobilisers in Category 1, it's just that they don't have the alarm included.

Thatcham Category 2-1

This is when you have an immobiliser and/or alarm upgraded or retrofitted to meet Thatcham Category 1 requirements.

For example, if you had a Category 2 immobiliser, you could upgrade it by having a professional fit a Thatcham-approved alarm. This would make it a Category 1 status alarm.

Other categories

The other Thatcham categories range from 3-4 and are mainly mechanical devices or physical locks like steering wheel and gear level locks. Locking wheel nuts are also in this category.

Categories S5 - S7 are tracking devices which show where the car is after it's been stolen. S5 devices can track where your stolen vehicle is and remotely immobilise the car by stopping certain engine functions. Category S7 can locate the vehicle, but can't shut it down remotely.

How can I tell if my car has a Thatcham device?

It's difficult to tell if your car has a Thatcham device, but you can find out. A sure sign that you have a car immobiliser is if you have a flashing LED light on your dashboard. You might also find information on your immobiliser in your car's manual.

In some circumstances, there might be a sticker in your car's window, or it might be detailed into your car's service history.

You could also do a type of controlled test. For example, locking your car then attempting to open it manually. If the alarm goes off, you might have a Thatcham device. If all else fails, you can ask your mechanic to inspect your car.

Remember, if you have a Thatcham approved Category 2 device, it's an immobiliser only. It doesn't have an alarm fitted.

Sometimes you might see Thatcham approved Category 2 alarm, but it's only the immobiliser. If you had an alarm on your car, it would either be Category 1 or Category 2-1.

Do immobilisers and alarms affect my car insurance?

Lots of different factors affect car insurance, from where you live to your driving history.

But insurers look favourably on Thatcham-approved immobilisers because they're regulated by a security-focused organisation. It also reduces your risk of your car being stolen.

If you have factory-fitted or other security devices, let your insurer know - though some insurers might only offer a discount on insurance for a Thatcham-approved device.

How to upgrade your car's security

If your car was built before 1998, it probably won't have an immobiliser or alarm. You could get one fitted by a Thatcham-approved fitter. This can reduce your insurance prices, and also deter thieves from breaking into your car.

If you have a Category 2 immobiliser, you could get that upgraded to a Category 2-1. That means you'll have an alarm as well as an immobiliser.

It's important to get a Thatcham approved fitter to install any security devices on your car. Doing this means the device gets installed to a high standard, that's recognised by insurers.

You could also look into physical security devices like steering wheel locks, gear stick and wheel locks just to bump up your security even more.