Secret codes in a mobile helps user to check proper functionality of mobile, unlock hidden features ,or to diagnose the mobile. Such a code is also there in the mobiles of Nokia lumia series. The code will open a diagnostic tool and it give you access to test all the hardware of the phone if it is working properly or not. You can use this feature to check your phone to make sure that all hardware is working properly, such as cameras and dead pixels on your LCD screen and much more.
To access the diagnostic tools on Nokia Lumia 800 open Phone Dialer and enter the code ##634# this will trigger the tools menu and you will see the list of below tools:-
This code will work on all the models of Nokia lumia series like, lumia800, lumia710 etc.