If there is a smell coming from under the crown, especially after eating sweets, if there is pain in the tooth that starts for 1-2 seconds and continues, there is a high probability of decay under the crown. In such a case, your dentist should do an X-ray check to see if there is decay under the crown. In addition to the X-ray check (because sometimes decay cannot be seen on an X-ray), he/she can remove the crown and look under it.