An image forming apparatus that is capable of specifying a failed portion while preventing delay of an image forming operation appropriately. An image forming unit forms an image on a recording material. A detection unit detects an abnormality. A controller determines a type of an abnormality detected by the detection unit. The controller controls, in a case where the abnormality is detected by the detection unit, a start timing of failure diagnosis that specifies a failed portion that causes the abnormality according to the determined type of the abnormality.