From a PID manual:
The Auto-tuning function (also called self tuning) can automatically optimize
the PID parameters for the system. The auto-tuning function will use the On/
off mode to heat up the system until it passes the set point. Then let it cool
down. It will repeat this about three times. Based on the response time of the
system, the built-in artificial intelligence program will calculate and set the PID
parameters for the controller. If your system has a very slow response, the
auto-tuning could take a long time.
6.1 To activate Auto-tuning, press and hold ">" key until the AT indicator
starts to blink, which indicates auto-tuning is in progress. When AT stops
blinking, the auto-tuning is finished. Now, newly calculated PID parameters
are set to be used for the system. Please note that Auto-tuning is only for PID control mode (when outy is set at 1 or 2).
6.2 To stop the Auto-tuning, press and hold ">" key until AT indicator
stops blinking. Then, the previous PID parameters values are resumed.