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Can you elaborate? Does it deactivates immediately after you have activated it?
Have you tried restarting the phone by press and hold the power button and the volume up button at the same time?
In the later software updates you will not get an icon on the homescreen when NFC is active. Make sure that you have Android Beam activated to share content between phones. Go to settings > more... > Android Beam.
The NFC sensor is located in between the camera lens and the Xperia logo on the backside of the phone.