The following article, compiled by Brian Whittaker, forms part of a lecture presented to third year students at South Bank University, London in January 2004.


The anti-theft system deters the theft of the vehicle with both active and passive systems. The active system monitors the vehicle perimeter and, in some markets the vehicle interior, for intrusion. If the system detects intrusion, an alarm sounds. The passive systems include a vehicle immobiliser as well as protection against intrusion through "super " locking (deadlocking)the doors. The vehicle is immobilised whenever the ignition key is removed from the ignition switch. Super locking or deadlocking prevents the interior door handles from opening the doors. In addition, there are many features and functions that enhance system operation. It is possible to perform the various functions associated with the anti-theft system remotely. To accomplish this, the handset generates a radio frequency when the lock button or unlock button is depressed. This signal is transmitted to the Body Control Unit (BCU) via a receiver located in front of rear sunroof beneath the headlining. Precise alignment of the handset to the receiver is not necessary. The approximate operating range between the handset and the receiver is 10 metres (33 feet). It is important to understand the configuration of the system on each particular vehicle. There are many different combinations of features with dedicated operating modes.

Anti-theft alarm and engine immobiliser systems

The modern vehicle is fitted with a sophisticated electronic anti-theft alarm and engine immobilisation system. It applies mainly to vehicles younger than five years and is normally fitted at the place of manufacture by the vehicle manufacturer. The vehicle key/handset will contain a coded transponder, which sets the vehicle alarm system and prevents the engine from being started unless a key with the correctly coded transponder is used in the ignition barrel. Both systems can be set independent of each other depending on the system.

Anti-theft alarm indicator light

A RED indicator light on the instrument panel or offside door flashes rapidly while the alarm system is arming itself. After a short period, the indicator light adjusts to a slower frequency, and continues flashing as an anti-theft deterrent until such time as the alarm / immobiliser is disarmed. Add-on alarm systems are freely available on the open market and these would normally be fitted to the older vehicle.

Alarm Systems

An alarm system may consist of the following:

  • Perimeter protection: This part of the alarm system protects the doors, bonnet and boot lid - the alarm will sound if any one of these apertures is opened improperly.

  • Interior protection: Also known as volumetric protection, this part of the system protects the space inside the car, by detecting movements within the passenger compartment or intrusions through the windows or hood, any of which will cause the alarm to sound.

  • Superlocking / Deadlocking: Includes all of the above features and, additionally, disables the sill locking buttons, thereby prohibiting operation of the door locks (from outside or inside the car).

Engine immobilisation

Features of the engine immobiliser system:

  • Engine immobilisation is an important aspect of the car's security system, it occurs automatically whenever the key is removed from the ignition barrel or whenever the car is locked (with handset or key).

  • The system could also include a feature known as passive immobilisation, whereby the engine is immobilised automatically thirty seconds after the starter switch has been turned off and the driver's door opened and closed -even if the driver leaves the key in the starter switch, or forgets to lock the doors and arm the alarm!

  • The engine is re-mobilised by a signal transmitted from the key or handset to the starter switch. This occurs automatically whenever the key is inserted into the starter switch and turned to position II, provided the key or handset is in close proximity to the switch.

Vehicle Recovery

Whenever a stolen vehicle, less than five years old is recovered, a short inspection prior to moving the vehicle would be advantageous for evidential purposes. In all cases the following points needs to be taken into consideration for continuity of evidence:

  • a. Note forced entry and ignition damage, if any, from outside the vehicle

  • b. Check position of all door locks

  • c. Check if any warning lights are flashing:- Ignition or alarm systems

  • d. Avoid disturbing the vehicle as much as possible

If the vehicle is locked, no attempt should be made to enter the vehicle or use the keys if available. As the vehicle has been locked by some method it is essential to determine how this has occurred. (Key or Handset) All Keys should be obtained as soon as possible to deter the owner from forwarding the key to their insurance companies and complicating the issue. The keys should be placed in special property for the vehicle examiner The SOCO should not enter the vehicle until after the vehicle examiner has completed his examination. The opening/unlocking of the vehicle will alter the condition of the alarm/immobiliser systems and prevent an accurate assessment of how the vehicle had been stolen or left locked.

If the vehicle is found unlocked, no attempt should be made to start the engine as some vehicles allow the engine management system to record which key has previously started the vehicle. The key available may not have been the last key to be used.

In all cases, continuity of evidence is essential especially as the recovery agent is between the officer's initial look and the examination carried out by the vehicle examiner.


Most modern keys are three keys in one:

  • A mechanical key will release the steering lock.

  • A coded 'electronic transponder chip', is read by the car when the keys inserted in the ignition.

  • A remote control will unlock the doors and turn off the alarm.

These keys are secure but are also expensive and time consuming to replace if lost or broken. Treat keys as you would expensive jewellery -keep them in a Safe & secure place.

Transponder keys
Master keys
Rolling codes
Remote controls
What if the remote doesn't work?

Transponder keys

  • Electronic, coded transponder chips embedded in the plastic body of the key were introduced from 1995. The chip is passive, so it doesn't need a battery -the code is read when you turn the key in the ignition.

  • If the transponder chip is broken or missing the engine won't start.

  • You will need to return to the dealer if you wish to replace your key. The dealer will have to re-programme the immobiliser's control unit to recognise the new key code.

Master keys

    Many early cars were supplied with a 'master key' (often red), which was not intended for normal use. The dealer uses the master key to programme a new or replacement key for the car.
  • Unfortunately, if you damage or lose the master key it could cost hundreds of pounds to replace. You may have to replace the complete engine management system costing more than 1000.

  • Car manufacturers have virtually stopped using master keys. They now hold car-specific security information in a central database, which the dealer uses when reprogramming the car and a replacement key.

  • If you're buying a used car, check the handbook. Make sure you get all the keys including a master key if necessary.

Rolling codes

    Early transponders used 'fixed codes' -the key sends exactly the same coded signal every time it's used.
  • Keys with 'rolling codes', which means the transponder code changes every time the key is used were introduced from 1999 and are now very common.

  • These should be virtually impossible to copy. They offer improved security but they're even more expensive to replace if lost.

  • Time and cost vary from manufacturer to manufacturer -expect to pay around 100 and wait up to three days for a replacement key.

  • The key might be even more expensive to replace if it includes a remote control for operating central locking and the alarm.

Remote controls

    More that 90% of all new cars are now supplied with a remote control to unlock the doors and turn off the alarm. Very convenient but not without their problems.
  • Some use infrared but most use a radio transmitter to send a coded signal to a receiver on the car.

  • The operating frequency (418Mhz or 433.92Mhz) is close to those used by MoD communication, radio amateurs and other common applications.

  • Interference can occur and in extreme cases the car can't be unlocked.

  • The AA helped to establish the Radio Activated Key Entry (RAKE) Committee in 1997 to ensure that the problem was recognised and researched. Car manufacturers were encouraged to improve the design of their systems to make them less susceptible to interference.

  • Modern cars are generally now much less likely to suffer from radio interference but the problem remains for older cars, particularly those built before 1995.

What if the remote control doesn't work?
1. Check the battery is not flat.
2. If you suspect radio interference try using the remote control close to your vehicle.
3. In extreme cases, AA patrols have had to tow vehicles away from interference, so the remote can work.
4. Cars with remote central locking should have a bypass system using the normal metal key to unlock the doors without setting the alarm off. This 'auxiliary entry' system should be explained in the handbook.
5. Having said that, the handbook will probably be locked in the car when the remote fails - familiarise yourself with the procedure now.


Immobilisers are an electronic device, usually connected to the engine-management computer that prevents the car from starting unless the correct key is used. The ignition key has a computer chip embedded in the head which "talks" to the car's engine computer. If the two don't talk, or send the correct message, the car won't start.


The use of vehicle transponders was introduced as a result of the German Insurance bodies refusing to underwrite vehicles that did not have some form of passive electronic immobilisation. The passive meaning that no action had to be undertaken by the user to 'arm' the system. A transponder system comprises two primary components being the processor/interrogator and a microchip (transponder) embedded within the head of the key. The processor usually interacts with the fuel injection and varied other vehicle electronics systems. When an attempt to start the vehicle is made, the interrogator sends a burst of Radio Frequency (RF) energy - at a defined frequency - to an antenna, which usually surrounds the face of the ignition cylinder. As the transponder contains a tuned circuit matched to the frequency of the RF burst, a small voltage is generated by induction within the transponder chip. Whether this charges a capacitor I don't know, but the result is that the induced voltage causes the transponder chip to emit, by RF, it's individual code number - a thirty two (32) digit number, which can also contain Alfa-numeric characters. If the processor recognises the returned code number, it then allows the starter motor, fuel pump and/or fuel injection systems to function, and allow the vehicle to start. A common sign of a faulty or absent transponder is that the vehicle will start but will only run for a few seconds. It is the processor/interrogator module that is programmed with which key is allowed start privileges. The transponder chip is only an identification number for the key in which it is embedded. Interestingly, the amount of time it takes to start, send and stop the transmission of the code number is a factor as to whether the processor will accept the code.


The main players in transponder technology seem to be Philips, Texas Instruments and Megamos, which each have their own idiosyncrasies regarding replacement keys and such. Some transponder keys can be duplicated using reader/encoding hardware and software and writable transponder chips. Others have to be introduced (programmed) into the processor/Interrogator to be functional. Several key blank manufacturers, Silca in particular, are devoting a considerable amount of R&D into transponder technology to enable the locksmith industry to offer a viable service to customers who's vehicles have transponder immobilisers fitted. With the exception of BMW, Mercedes, Volvo and a few other models using "crypto" or rolling code transponders, the 'average locksmith' is able to provide key duplication, and generation in some cases, on a competitive basis without manufacturer or dealer involvement. Some systems require the use of a 'learning key' to program additional keys - If a customer presents with a broken key, it is possible to duplicate or generate a new key using a normal key blank and then hold the broken key's head close to the ignition cylinder to allow the vehicle to start. Another method is to carefully remove the transponder chip from the key head and epoxy the chip to the key. Of course using the correct OEM or transponder key blank is the ideal solution. Having two transponder keys on the same key ring or in close proximity to the ignition cylinder when starting the vehicle can cause both keys to emit their codes at the same time, and cause the processor/interrogator to fail to accept either code. - Corrupted data so to speak. As to whether having a "toll road transponder" interfere with vehicle immobiliser, it is possible, but unlikely, as the "toll transponder" would probably operate at a different frequency. Some of the immobilisers can be disabled by use of the vehicle system diagnostic tool, but I feel that this function is for diagnostics rather than disabling the immobiliser completely. The technology is fairly new and the manufacturers are being VERY protective of the information on bypassing the systems.

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