1.The heating tube connection is too loose. Please tighten the connection.

2. Heating tube/ temperature controller/ power tube damage.

Please troubleshoot by following the below methods:

1) Open the machine case, and find the connectors of two heating tubes, exchange the  two connectors, heat the nozzle again. If the failed one can work now, it means one  heating tube is broken and needs to be replaced; 

2) If after exchanging the two heating tube, the failed nozzle still can’t work, please  exchange the heating tubes back, and try to exchange the connection of the two  temperature controller, heat the nozzle again, if the failed nozzle can work, it means  one temperature controller is broken and needs to be replaced; 

3) If the above cannot solve the problem, it means it should be the problem of the  power tube on the main board, please replace it.