If the thread breakage sensor in the thread tension disc of the thread tension knob cannot detect that the thread is being fed, even if the machine is running, a broken thread error occurs and the machine is stopped.
If the thread came out of the thread tension disc, even though the thread is not broken, the machine detects that the thread is not being fed and stops.

Clean the tension unit.
Correctly pass the thread through the thread tension disc.

If the broken thread error occurs after the machine is cleaned and rethreaded, the sensor may be damaged.
Consult your nearest authorized Brother dealer.