There are 2 basic troubleshooting techniques that can fix a variety of trivial problems.

The first is to reset the Contro-3 by pressing the reset button. This is a plunger-style button located on the back of the unit (upper-left when facing display). The Contro-3 will first show a title screen that helps to identify the program running inside. Then it will resume normal operation.

The second tip is to do a Refresh Scale on the PC program. This will clear the Contro-3's memory of the current data tables (not the transactions,though) and rebuild them by sending them all again.