Cartos are meant to be disposables.
I've had some luck cleaning them, either by boiling or soaking in isopropyl alcohol with the help of an ultrasonic cleaner. Maybe 90% of them survive such treatment -- once. After one cleaning, I'll pitch them, since they seldom are any good once the internal coil gets totally gunked, and there's no way to thoroughly clean that internal coil. Do a search: you'll find lots of threads about cleaning cartos.
One way to extend the life of a carto is to use it in a tank. I've about trebled their life. Then they get cleaned, again only once.
By contrast, I've had excellent luck with Vivi Nova and Vivi Nova Mini clearomizers. I can clean them repeatedly. If the coil in a Vivi head goes bad, I can replace it without having to chuck the whole Vivi. And, if you're at all handy, you can recoil a Vivi Nova head in about 30 seconds for a few pennies in material. Again, do a search to find about Vivi Novas.