Adding keys to the immobiliser system is easy on the Galaxy providing you have two unique keys. (This is one area where the Sharan and Alhambra is different so this procedure doesn't apply)

How the system works:
Most modern immobilisers work by reading a chip (called a transponder) that's built into the key. When the ignition is switched on the reader unit around the ignition barrel emits radio waves which energise the chip inside the key. This in-turn then transmits it's code for the reader to read - if it matches one of the codes stored in the immobiliser controller the car is allowed to start. Since the transponder is powered by radio waves there is no battery involved and the only batteries in the key are purely for the remote unlocking side of things.

Unique or Cloned keys:
Unique keys contain a transponder with an individual code whilst cloned key's (normally made by locksmiths and the like) contain a transponder that transmits the same code as another, cloned, key. From new all keys supplied would contain unique transponders.

Original Equipment and Key type:
A= Two Button Remote Key
B= Regular Key
C= "Valet" / Emergency Plastic Key

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The Galaxy originally shipped with 3 keys.
LX and Zetec trims came with two regular (B) keys and an emergency key (C)
The Ghia came with one of each.

This was revised around 2003 (53) with all models coming with one of each key type.

All three types of key are capable of being coded to the immobiliser system and all should have been coded from the factory however sometimes the valet key wasn't so if you have the valet key then check it starts the car and if not follow the procedure below!

Coding Procedure:
You need two Unique keys for this to work, cloned key's won't work! Your new key needs to have an ID44 immobiliser transponder in it (The only Ford model to use this chip) and already have the key shaft cut to fit the locks.

The following steps need to be carried out within 5 seconds of each other:

1) Put Key 1 into the ignition and turn to Position 2 (Ignition On or the one before start)
2) Wait 1 second and then turn off and remove
3) Put Key 2 into the ignition and turn to Position 2
4) Wait 1 second and then turn off and remove
5) Put the new key into the ignition and turn to Position 2
6) Wait 2 seconds and then turn off and remove

Remote key not starting the car after battery change:
The transponder on keys B and C is embedded into the plastic so it can't be lost. Unfortunately on the remote key it sits in a channel within the key shaft which means that there is a possibility it will fall out when changing the batteries in the remote. The photo below shows where the chip should be!

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The chip is small in size and can fall out, unfortunately without it the key cannot be used to start the car.

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What if I don't have two unique keys?
There are only really two options if you don't have two unique keys. The First is to take the car to a Ford Dealer who can code you a new key for approximately Ã,£120. The second is to have a locksmith clone your one remaining key so you have two to start the car with for around Ã,£50. Whilst a fair bit cheaper the downside of this is you still don't have two unique keys and can therefore not code a replacement or third key.

It is highly recommended that if you only have two unique keys you obtain and code a third! Else, if you do happen to loose a key you won't be able to replace it!