Universal Robots Forum

C4A12 Connection with joint lost


I have a customer that getting Error code C4A12 “Connection with joint lost” on joints 0 and 2 on their UR5 CB3. They have tried to do a complete rebooting sequence and checked that the communication cables are connected properly.

The robot can still runs as normal for several hours but the error code keeps coming back, sometimes after only a few minutes.

Is there reason to believe that the joints are “broken” and need to be replaced or is it possible that there is an easier solution?


I would check the cable from the robot to the controller and make sure it’s both fully seated and locked in place (the cable has a twist-lock mechanism on the end hooking into the controller). If that doesn’t work I’d recommend creating a case in myUR for the Service team to investigate further. You can go to the following link and hit “sign up now” if not already registered: