Car keys

• Stolen while the owners were asleep at night
• Stolen while owners have been in the garden or have 'popped out for five minutes'
• Left in front door locks
• 'Fished' through the letter box or through open window fanlights
• Taken following comprehensively burglery of the home, and the family car used as a getaway vehicle
• Stolen from a family's home while they were on holiday and used to take three cars from a locked garage
• Stolen from workplaces, gym lockers and changing rooms
• Quietly picked out of unwatched bags or pockets
• Stolen by way of threats, muggings or carjackings

Different types of keys

Most modern car keys are three keys in one.

• A mechanical key to release the steering lock
• A coded 'electronic transponder chip' read by the car when the key is inserted into the ignition
• A remote control to unlock doors and turn off the alarm

These keys are secure but are also expensive and time-consuming to replace if lost or broken.

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 want to replace your key. The dealer will have to reprogramme 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 £1,000.

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 one was supplied when new.

Rolling codes

Very early transponders used 'fixed codes' – the key sends 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

Virtually all new cars are supplied with a remote control to unlock the doors and turn off the alarm. This is very convenient, but not without its 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 the worst cases the car can't be unlocked.

Modern cars are 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?

• Check that the battery in the key isn't flat.
• If you suspect radio interference try using the remote control close to your vehicle.
• In extreme cases, AA patrols have towed cars away from interference, so the remote can work.

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 but the handbook will probably be locked in the car when the remote fails – familiarise yourself with the procedure now.

(updated 3 May 2012)