A diagnosis system includes circuitry configured to form an image to be used for diagnosing the image forming apparatus, receive designation of a first diagnosis condition for detecting abnormality of the image formed by using the image forming apparatus, scan the image formed by the image forming apparatus, using a scanner, extract feature information representing the abnormality from the scanned image in accordance with the designated first diagnosis condition, diagnose the abnormality of the image forming apparatus based on the extracted feature information in accordance with the designated first diagnosis condition, and determine, in response to receiving designation of a second diagnosis condition different from the first diagnosis condition after performing a diagnosis under the first diagnosis condition, whether the feature information extracted for the diagnosis under the first diagnosis condition is usable for a diagnosis under the second diagnosis condition.