The immobiliser has been around in some fashion or another for many years. In concept it is designed to stop unwanted borrowing of your car. The early immobilisers worked by cutting into the LT circuit of the ignition or even the power feed to the starter motor. These early types were very crude and often had an interior hidden switch.
The immobiliser can also work in variable ways, The common systems can immobilise the starter motor, fuel pump or indeed the ignition system some do all.
As technology has improved so indeed has the car immobiliser. Later aftermarket
immobilisers used far more technical tools for the systems to work including
electronic contact switches which operate an electronic control unit which then
allows the engine to start.
Car makers use many different types of immobiliser some use key pads which require the driver to enter a code before the engine will run.
Others use a coded key system which is inconspicuous and
excellent in crime prevention.
These keys have an internal electronic chip with which the car communicates.
Once the car accepts the keys code it will allow the engine to run.
The coded keys however can fail and if they do they can be expensive to
repair.
Some makes such as Audi, BMW, Vauxhall, VW etc use coded keys which are programmable by
their appointed agents who can cut you a key and program it to your car making
it work again.

Other coded key systems are supplied with a red or master key
when new. But if you loose the master key you cannot cut any others and have
them coded to start your car.
Ford, Rover, Honda etc use this system but their master keys are not too expensive to replace. An
appointed agent can supply a new mater and re programme it to your car.

Fiat and Alfa Romeo are without a doubt probably the most
expensive systems if you require a new master key.
You may have to re-mortgage your home if you require a new one.
Alfa and Fiat can charge a great deal over £1000.00 depending on the model of your car.
We do say that the UK has an
open economy but this has to be a monopoly which needs looking into. And one
which Alfa and Fiat should have
removed.
The system itself works well but the after market support is awful. I for one had to buy a red key for an Alfa
and I cannot express enough my disappointment at
their rip off prices and very poor performance.