Lost your only car key?What happens next

First: is it lost, or is it locked inside?

These are two completely different jobs and two completely different costs. If you can see the key through the window, you need the car opened — that is quick. If the key is genuinely gone, a new one has to be cut and coded from scratch.

Check the obvious places before you phone anyone. Coat pockets, the boot, the ignition, and with whoever drove last.

What a locksmith will ask you

• Make, model and year. Off the licence disc if you are not sure. This decides whether the job can be done at all, and whether it can be done on site.
• Whether any working key exists. Even a spare in a drawer at home changes the job completely and usually makes it cheaper.
• Where the vehicle is. At home, at work, or standing in a parking lot somewhere.
• Proof it is yours. Your ID plus a licence disc or registration papers.

Cutting versus coding

A modern car key does two jobs. The metal blade turns the lock, and a transponder chip inside the plastic head tells the immobiliser that the car may start. A key that is only cut will open the door and do nothing else. Both halves have to be done, and the coding is the part a hardware shop cannot do.

If the vehicle uses a proximity or push-start system, there is no blade to cut at all — the whole unit has to be programmed to the car.

When you have to go to the dealer

Some manufacturers lock key data behind their own systems, and on certain models the only route is the dealership, usually with the car towed in. A straight locksmith will tell you this on the phone rather than get the vehicle apart first and then discover it. If your car is one of those, you should hear it before anyone arrives.

The cheapest version of this problem

Get a spare cut while you still have a working key. It is a fraction of the work — the existing key supplies the code and the car is already there to pair against. Almost everyone who phones about a lost key says the same thing afterwards.