Ford Focus Owners Manual: Principle of operation

Engine immobiliser

...

Coded keys

Note: Do not shield your keys with metal objects. This may prevent the receiver from recognising your key as a valid one. Note: Have all of your remaining keys erased and recoded if you lose a key ...

More about Ford Focus:

Ford Focus Quarter Panel Speaker - Removal and Installation
  Item Part Number Description 1 18808 Quarter panel speaker 2 W701930-S Quarter panel speaker screws (4 required) 3 — Quarter panel speaker electrical connector (part of 14A005) Removal and Installation Remove ...