The START/STOP button on the face of the HAKKO preheaters sends a signal to the switching PCB that is located in the rear of the preheater unit near the cooling fan. If the airflow is restricted, heat may build up inside the unit and affect the function of the components on this PCB. As a result, the preheater will keep running and will not respond to the START/STOP button press.
In the event the unit is overheating, please stop the preheater using the power switch and allow the unit to cool before you resume using the preheater.
Please ensure that the preheater has sufficient open space surrounding it and that the cooling fan opening is not restricted as failure to do so can result in the issue described above.