Secret Codes helps you to access some internal functions of mobile which are intended for developers and troubleshooting purposes. They are the combination of numeric and Special Characters like * and #.These codes can give you access to the whole new world of hidden menu, diagnostics test and much more.
Secret Codes of HTC Desire 820
Step 1 : First of all open your keypad (where you dial the phone numbers)
Step 2 : Then dial the secret code *#*#3424#*#*
This will open up a HTC Function Test Program as shown in figure ( you need to click accept to proceed)
Then select the test you need to perform and press the Run key.
These codes will help you to check various hardware parts of your mobile if they are working fine or not.
Hidden Menu In HTC Desire 820
One more code that you can try on you HTC mobile phone is *#*#4636#*#*
By dialing this code on the mobile you can get information on phone, battery, wifi and some usage statistics like shown in the image below (just click the menu to explore)
There are plenty more of these to be found on the internet, these are just some of the best ones.
CAUTION: Don’t mess unless you know what you are doing.