What a Car Immobiliser Is and How It Works
A car immobiliser is an electronic security system that stops the engine from starting unless it recognises a coded key, and it works by reading a transponder chip in the key each time the ignition is switched on.
Inta-Lock Car Key Guides. Updated October 2026.
The chip in the key answers a reader beside the ignition, the immobiliser compares the answer with the keys held in its memory, and a match releases the engine. A key with the right blade and the wrong chip turns the lock and the engine stays off. European type approval rules have required an immobiliser on new passenger cars since 1 October 1998.
An immobiliser is not an alarm and not a tracker, and a factory immobiliser on a keyless car does not stop a relay attack on its own. The same memory of keys is the reason a new key is programmed to the car, and the reason a lost key is removed from that memory.
What Is a Car Immobiliser?
A car immobiliser is an electronic security system built into the car that keeps the engine from running until it recognises a key coded to that one vehicle.
The purpose is narrow. An immobiliser does not guard the doors and does not make a noise. It decides whether the engine is allowed to start, and it decides from the key and not from the lock.
On the cars built before it, a blade that turned the ignition lock, or two wires joined behind it, was enough to start the engine. With an immobiliser the turn of the lock is only a request, and the engine control unit waits for the immobiliser to accept the key.
A factory immobiliser has four parts, which are the transponder chip in the key, a reader coil at the ignition lock or the start button, the immobiliser module that holds the list of accepted keys, and the wiring between them.
How Does a Car Immobiliser Work?
A car immobiliser works by checking the key in five stages at every start, and the engine runs only when the last stage releases it.
| Stage of the start | Part doing the work | What happens | What the driver sees when the stage fails |
|---|---|---|---|
| The key is presented | The key and the reader coil | The key goes into the ignition lock, or on a push button start car it sits inside the cabin within reach of the reader | The dashboard reports that no key is detected |
| The chip is powered and asked for its code | The reader coil, also called the antenna ring | The coil puts out a small field that powers the transponder chip in the key and asks it to identify itself | The key is read on some days and not on others |
| The chip answers | The transponder chip in the key | The chip sends its code back to the reader coil | The security light flashes, because a damaged chip gives a weak answer or none |
| The code is compared | The immobiliser module | The answer is compared with the keys held in the immobiliser memory | A key that is not in the memory is rejected, although its blade turns the lock |
| The engine is released | The engine control unit | A match sends a release to the engine control unit, which lets the fuel and ignition systems run | The engine cranks and does not fire, or starts and stops after a moment |
The whole key check passes in the moment between the ignition being switched on and the engine being cranked, so a driver with a recognised key never notices the immobiliser. The system sets itself again when the ignition is switched off.
A key the immobiliser rejects, or a new key to add to the car in Leicester or Leicestershire? Matt does the work at the vehicle.
Call Matt on 0116 442 2084Does Every Car Have an Immobiliser?
No, not every car has an immobiliser, but European type approval rules have required one on new passenger cars since 1 October 1998.
A car first sold new after that date carries an immobiliser from the factory. An older car, an import and a car whose system was altered by a past owner are the cases where the answer is checked and not assumed.
Three signs show that a car has a factory immobiliser.
- A security light on the dashboard. A key or padlock symbol lights when the ignition is switched on and goes out once the immobiliser accepts the key.
- A key with a plastic head or a remote. The transponder chip sits inside the head of the key or inside the remote, so a key that is all metal carries no chip.
- An entry in the handbook. The handbook of the car names the immobiliser in its security or starting section and shows the symbol the dashboard uses.
Where the three signs are unclear, the handbook of the car settles the question, and for key work the immobiliser a given car carries is confirmed on the call.
Why Does a New Key Need Programming to the Immobiliser?
A new key needs programming because the immobiliser releases the engine only for keys held in its memory, and a freshly cut key is not yet one of them.
Cutting and programming are two separate jobs. Cutting shapes the blade so that it turns the locks. Programming adds the transponder chip of the new key to the list the immobiliser accepts.
The job of programming a car key is carried out at the vehicle. The route into the immobiliser memory differs by make and model year, and what applies to a given car is confirmed on the call.
Makers protect the information that key and immobiliser work relies on. SERMI is the scheme that approves independent operators for access to security related repair and maintenance information, and Matt East of Inta-Lock is SERMI accredited.
Identity and authority to request the work are checked before a key is programmed. The V5C vehicle registration certificate, which names the registered keeper, and photo identification are common examples, and what applies is confirmed on the call.
How Does an Immobiliser Differ From an Alarm and a Tracker?
An immobiliser stops the engine from being started, an alarm warns that the car is being entered, and a tracker reports where a stolen car is, so each one acts at a different point of a theft.
| Device | What it acts on | What it does | What it does not do |
|---|---|---|---|
| Immobiliser | The engine and the systems that run it | Keeps the engine from running without a recognised key | It gives no warning and does not keep a thief out of the cabin |
| Alarm | The doors, the bonnet, the boot and the cabin | Sounds a siren when the car is opened or disturbed while set | It does not stop the engine from being started |
| Tracker | The location of the vehicle | Reports the position of the car after a theft so that it is found | It does not prevent the theft itself |
The split is the one used for aftermarket products under Thatcham Security Certification, where Category 1 is an electronic alarm and immobiliser, Category 2 is an electronic immobiliser, and separate categories cover vehicle tracking systems.
Does an Immobiliser Stop Keyless Car Theft?
No, a factory immobiliser does not stop a relay attack on its own, because the attack carries the signal of the real key to the car and the immobiliser accepts a key it recognises.
Thatcham Research states that a car with passive keyless entry and start is open to the relay attack, and its guidance on how to increase car security names a signal blocking pouch for every key, spares included, and a key that goes to sleep when it is not moving.
Against a thief with no key signal at all the immobiliser still does its job, which is why a copied blade or a forced ignition lock leaves the engine off.
Which Immobiliser Questions Lead to Other Work?
Four situations sit beside the working factory immobiliser, and each one leads to a different page and a different job.
How Does a Factory Immobiliser Differ From a Ghost 2 Immobiliser?
A factory immobiliser checks the key, while an Autowatch Ghost 2 immobiliser checks the driver. Ghost 2 is an aftermarket immobiliser installed into the CAN bus network of the vehicle, and the vehicle does not drive until a disarm sequence is entered on buttons the vehicle already has.
The two work together. The key is still coded to the factory immobiliser, and the sequence is entered after the ignition is on. The sequence sends no radio signal, so it is a check that a relayed key signal does not satisfy. Whether a given vehicle is supported is confirmed on the call before Ghost 2 immobiliser fitting is booked.
Why Does an Immobiliser Reject Its Own Key?
The opposite of a working system is an immobiliser that refuses a key it accepted the day before. The engine cranks and does not fire, or the security light keeps flashing with the key in the ignition.
The fault sits in the key chip, the reader coil, the module or the wiring, and a spare key that starts the engine places it in the first key. Finding which of the four has failed is the work of immobiliser diagnostics, carried out at the vehicle.
How Does the Key Type Change the Reading?
A transponder key is read at the ignition lock, a remote key adds door buttons to the same chip, and a keyless key is read from inside the cabin. The signs that tell one from another are set out for anyone working out which type of car key you have.
What Happens to a Lost Key in the Memory?
A lost or stolen key stays in the immobiliser memory until it is taken out, so it still starts the engine. When replacement keys are made, missing electronic keys are removed from the immobiliser memory where the vehicle supports it.
A mechanical blade still turns a physical lock unless that lock is changed or rekeyed.
Immobiliser and key work in Leicester is carried out at the vehicle, and the call to Matt is answered 24 hours a day.
Frequently Asked Questions
Where is the immobiliser in a car?
The immobiliser is spread across four places in a car, which are the chip in the key, the reader coil at the ignition lock or start button, the immobiliser module and the wiring between them.
Depending on the maker, the module is a unit of its own or part of the instrument cluster, the body control unit or the engine control unit.
Is an immobiliser the same as a car alarm?
No, an immobiliser stops the engine from starting and an alarm sounds when the car is entered or disturbed.
Many cars carry both, and the key works both, which is why the two are often taken for one system.
Does a flat remote battery stop the immobiliser reading the key?
No, on a key with a passive transponder chip a flat remote battery does not stop the immobiliser, because the reader coil powers the chip.
The remote buttons stop working and the key still starts the engine from the ignition lock. For a keyless key with a flat battery, the handbook of the car shows the backup way the maker provides to start the engine.
What does a flashing immobiliser light mean?
A flashing immobiliser light with the ignition on means the immobiliser has not accepted the key.
On many cars a slow flash with the ignition off and the car locked is the normal sign that the system is set. The handbook of the car shows which pattern applies.
Does an immobiliser lower the cost of car insurance?
The effect of an immobiliser on a premium is decided by the individual insurer, and no fixed saving applies.
Thatcham Research states that its security certification does not by itself secure recognition from an insurer, which is agreed between the product maker and its insurance partners.
Is a factory immobiliser switched off or removed?
No, a factory immobiliser is part of the control units of the car and is not a part the driver switches off.
A system that refuses its key has a fault in the key chip, the reader coil, the module or the wiring, and what the repair involves for a given car is confirmed on the call.
How much does it cost to add a key to the immobiliser?
The cost of adding a key to the immobiliser is quoted for the individual vehicle, and a quote is agreed before work begins.
The key type, the make and model year, and whether a working key exists all move the figure. A car with no working key involves more work than a car with one. No price list is published.