Well, they’re not really “secret”. Since we know about them…

These codes might not work for carrier/network phones, but most codes work. These all works on regular One M7/M8 that I’ve tested on.

You simply enter the codes below in the dialer (where you make phone calls).

Show IMEI

*#06#

HTC Function test

A function test where you can test functions, good for troubleshooting.

*#*#3424#*#*

Phone information and testing menu

This is very useful since you got temperature for battery under Battery information.

*#*#4636#*#*

These are the most common (and working) codes. If there’s more confirmed ones: Let me know.