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#*#*