Central locking not working on my car?

I have a ford focus and the locking is only working when i use the key in the driver door, it wont lock however when i press the locking button on the key, i dont think its a problem with the key because when i try to lock the passenger door from inside the car it wont lock, could this be an electrical fault that i can fix myself???|||Remote battery is dead or faulty.|||so the remote doesnt work at all right, but the key does?? have you checked the batteries in the remote?