Testing Mode is a menu on your FP1 where you can test the functionality of lots of the phone’s hardware. This is great to distinguish between hardware or software issues.
One important example is the #touchscreentest, but you can test almost all hardware functions.
To get there turn off your FP1, unplug all cables, press and hold the volume down button and the power button simultaneously (it helps to start pressing the volume down slightly earlier).
In Testing Mode you navigate by:
- Power selects
- Volume down moves you down
- Volume up (sometimes) gets you to the previous menu
- The “Back” softbutton moves you up
On the FP2 there is no testing modes but you have #dic:checkup which provides similar tests in Settings → Maintenance. There you also have a tool to recalibrate the proximity sensor, which is responsible for your screen turning dark while you hold the phone to your ear during a call and lighting back up if you pull it away again.