How does a car immobiliser work? The key check, stage by stage

Every time you start your car, the immobiliser runs a check you never notice. LockCAR builds CAN immobilisers in Birmingham, and this page walks through each stage of that check, what you see when one fails, and where an added immobiliser fits in.

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Five stages
Checked at every start
Re-arms itself

Animation of the LockCAR second check: without the TAG, the engine does not start — The extra stage: after the key check, the engine still waits for your TAG.

The five stages of the key check

The engine runs only when the last stage releases it.

What happens between switching the ignition on and the engine firing.
Stage	Part doing the work	What happens	What you see if it fails
1. The key is presented	The key and the reader coil	The key goes into the ignition lock, or sits in the cabin on a push-button car	A message that no key is detected
2. The chip is woken	The reader coil, also called the antenna ring	The coil sends out a small field that powers the chip and asks it to identify itself	The key is read on some days and not on others
3. The chip answers	The transponder chip in the key	The chip sends its code back to the coil	The security light flashes, because the answer is weak or missing
4. The code is compared	The immobiliser module	The answer is checked against the keys held in its memory	A key that is not on the list is refused, even if its blade turns the lock
5. The engine is released	The engine control unit	A match sends a release, and fuel and ignition are allowed to run	The engine cranks but does not fire, or starts and stops after a moment

What you notice when it works

Nothing at all. The whole check passes between the ignition going on and the engine cranking, so a driver with a recognised key never sees the immobiliser at work.

When you switch off, the system sets itself again. You never arm a factory immobiliser by hand.

Many newer cars use a question and answer that changes each time, so a recording of one start does not help a thief at the next.

If your car shows a key warning or will not start, read our guide to immobiliser problems before you assume the system has failed. A flat fob battery is a common cause.

Where the factory check can be fooled

Each method gives the car what it is waiting for: a key it believes.

Relay attacks: Thieves carry your keyless fob’s signal from the house to the car. Stages one to three pass as if the key were beside the car.
Cloned or newly coded keys: A key that thieves have added to the module’s list passes stage four like your own.
Attacks on the wiring: Some thieves reach the car’s data network and send messages that imitate a genuine key.
It checks the key, not the driver: The system has no way to tell who is holding the key. Anything it accepts as the key will start the car.

How a LockCAR immobiliser adds a sixth stage

A second proof, after the keyOnce your key passes, the engine stays blocked until you show your TAG, smart ring or smartwatch, or enter your PIN on the steering wheel buttons.
Independent of the car keyA relayed or cloned car key does not carry that second proof.
Works on 12 V and 24 V vehiclesCheck yours with the Vehicle CHECK before you book.
One paymentThere is no subscription. You pay once for the kit and the fitting.

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Add the sixth stage to your car

Check your car first

Enter your registration in the Vehicle CHECK. LockCAR immobilisers work on 12 V and 24 V vehicles, and the check confirms yours before you spend anything.

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Book an approved installer

Fitting at your home, at work or at an installation centre. A TAG immobiliser costs £249 fitted and the PIN code version costs £299.

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Questions about the key check

Does the key need to be in the ignition?

On older cars, yes: the reader coil sits around the ignition lock. On push-button cars the key only needs to be inside the cabin, within reach of the reader.

What is a transponder key?

A key with a small chip inside. For the immobiliser check, the reader coil powers the chip for a moment and reads its code.

Why does my car crank but not start?

If the check fails at the last stage, the engine control unit holds back fuel or ignition, so the engine turns over without firing. Other faults cause the same symptom, so start with the fob battery and see common immobiliser problems.

Does an immobiliser work when the car is unlocked?

Yes. It does not care whether the doors are locked. It checks the key at every start.

Do I keep using my normal key with a LockCAR immobiliser?

Yes. You use your car key as usual. The LockCAR immobiliser adds its own check on top, with a TAG, smart ring, smartwatch or PIN.

Does the check slow down starting?

Not in a way you would notice. With the PIN code version, you add a short button sequence before you drive.

Why does my key need a chip if it has a blade?

The blade only turns the lock. The chip is what the immobiliser checks, so a copied blade without the right chip turns the lock but the engine will not run.

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.