I don't think hot glue expires. You do have to let the gun heat up and use the right type glue in it. The older the gun the longer it could take to heat because of age. I have had my gun for about 15 years and my glue for about 10 years. I don't use very often anymore, but it still works. It just take a little longer to heat up and melt the glue. If you are having a problem with yours replace your gun.