Crater nozzle, default 0.4 caliber
Clogging, wear caused by abrasive material, or need for a new nozzle of a different size
Socket tool
Open end wrench
New nozzle
If there is any residual material in the nozzle, heat the hot end to melt the material.
Be careful of high temperatures during replacement to avoid burns.
Check the heating block after replacement to prevent it from loosening during the nozzle replacement process.