The PH-NM heating nozzle can be easy to backfire due to its simple design. Should you not shutdown quickly enough the heating nozzle insert can become damaged.
The nozzle insert is very simple to replace, unscrew the PH-NM end cap, knock out the old damaged heating insert. Check the end cap is not damaged, if it is replace the complete nozzle.
The insert has two diameters, the smaller diameter should face out of the end cap. With the new insert fitted to the end cap screw back onto the body of the heating nozzle.