The Nokia Lumia series running windows phone does not support any of the Nokia secret codes we are familiar with, since the phone runs windows phone it has an entirely different architecture. At this stage there is only one major code however it does unlock several features and options, additionally on the latest software you only have to enter the code once and the new application will remain on your menu.
To access the hidden diagnostics menu on your Nokia Lumia simply dial ##634# on your handset and you will be taken to the menu, the applicaiton will now also permantly appear in your applications menu. The following features are available to you in the menu;
Depending on your needs the most useful function here will likely be the battery status function which allows you to see your current power usage on the phone, although it does not provide detail on which applications are using power it is a great way to see if you are drawing more power than you should due to an application or network issue. Anybody that owns a Lumia 800 will likely have experienced battery drain issue more than once, this tool helps you see if you have solved the issue.