What is a car immobiliser? A plain guide for UK drivers

LockCAR designs and makes CAN immobilisers in Birmingham, so this is the question we answer most. This guide explains what an immobiliser does, the parts inside your car, the gaps thieves use, and how an added immobiliser gives your engine a second check.

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LockCAR no TAG, no start animation: the engine stays off until the TAG is present — No TAG, no start: an added immobiliser asks for proof your car key cannot give.

An immobiliser in one minute

An immobiliser is an electronic system built into the car. It keeps the engine from running until it recognises a key coded to that one vehicle.

Its job is narrow. It does not guard the doors and it does not make a noise. It decides whether the engine is allowed to start, and it decides from the key, not from the lock.

On cars built before immobilisers, a blade that turned the ignition lock, or two wires joined behind it, was enough to start the engine. With an immobiliser, turning the key is only a request. The engine control unit waits for the immobiliser to accept the key before it lets fuel and ignition run.

The system sets itself again every time you switch the ignition off, so there is nothing to remember and nothing to press.

The four parts of a factory immobiliser

You will not see any of them, but every start depends on all four.

The transponder chip in the key: A small chip that holds a code belonging to your car. The reader powers it for a moment, so it works even with a flat fob battery on many cars.
The reader coil: A ring of wire around the ignition lock, or near the start button on push-button cars. It wakes the chip and reads its answer.
The immobiliser module: It keeps the list of keys your car accepts and compares every answer with that list.
The wiring and the engine control unit: When the module accepts the key, it tells the engine control unit to let the engine run. If not, the engine stays blocked.

To see each stage at the moment of starting, read how the key check works, stage by stage.

What a factory immobiliser stops, and what it misses

A factory immobiliser trusts the key. If a thief can make the car believe the key is present, the engine starts.

It stops

Hot-wiring behind the steering column
A copied blade with no chip
A key that was never coded to the car

It misses

A relay attack that passes your real key’s signal to the car
A cloned key, or a new key coded by thieves
Towing or lifting the car away
Break-ins to take what is inside

How an added immobiliser gives your car a second check

Separate from your car keyA LockCAR immobiliser keeps the engine blocked until you prove it’s you, with a TAG, a smart ring, a smartwatch or a PIN.
No fob, no startA relayed or cloned car key on its own is not enough to start the engine.
Fitted where you areLockCAR approved installers fit it at your home, at work or at an installation centre.
Paid onceThere is no subscription. You pay for the kit and the fitting, and that is it.

Is your car ready for a second immobiliser?

Check your car first

LockCAR immobilisers work on 12 V and 24 V vehicles. Enter your registration in the Vehicle CHECK to confirm yours before you book.

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Book your fitting

Kits start at £139. A fob immobiliser fitted by an approved installer costs £249, and the PIN code version costs £299.

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Questions drivers ask about immobilisers

Do all cars have an immobiliser?

New cars sold in the UK have had to come with one since October 1998. Older cars and some imports may not. Read how to tell if your car has one.

Can I see the immobiliser in my car?

Not usually. The parts sit behind the dashboard and inside the key. Many cars have a small key or padlock light on the dashboard that shows the system is armed.

Is an immobiliser the same as an alarm?

No. An alarm makes a noise when someone gets in. An immobiliser stops the engine starting. See immobiliser vs alarm vs tracker.

Why add another immobiliser if my car already has one?

Because the factory one trusts your key. If thieves relay or clone it, the engine starts. An added immobiliser asks for a second proof that the thief does not have.

How much does a LockCAR immobiliser cost?

Kits cost £139 for the fob (TAG), £149 for the smart ring, £160 for the smartwatch and £189 for the PIN code version. Fitted by an approved installer, the TAG is £249 and the PIN is £299.

What does Thatcham have to do with immobilisers?

Thatcham Research sets security categories that insurers often refer to. LockCAR immobilisers are going through Thatcham Security Certification, and we will publish the details when a certificate is issued.

Does an immobiliser stop a car being towed away?

No. It stops the engine starting, so a car lifted onto a truck is not affected. A GPS tracker helps in that case, and the LockCar One Plus 4G adds one to a dashcam and immobiliser, with fitting included for £499.

Written by the LockCAR engineering team. We design and build our immobilisers in the UK and fit them through our approved installers. Meet the engineers. Last updated 18 October 2026.