Image forming apparatus

A method of operating an image forming apparatus, the image forming apparatus including an image forming unit configured to form an image on a sheet, a fixer configured to fix the image formed on the sheet by pressing the image, a first temperature sensor, and a second temperature sensor, the method comprising:

• measuring an ambient temperature with the first temperature sensor, the first temperature sensor disposed in a first place away from the image forming unit and the fixer;
• measuring an ambient temperature with the second temperature sensor, the second temperature sensor disposed in a second place closer to the image forming unit than to the first temperature sensor;
• storing in a storage one or more difference values, each difference value being a difference value between a first temperature and a second temperature, the first temperature measured by the first temperature sensor and the second temperature measured by the second temperature sensor, when a switching condition is satisfied for switching to an intermittent mode for intermittent printing in which printing stop and printing are repeated;
• and
• storing information indicating that a reference value for determining an abnormality is exceeded, when a difference between (i) a statistical value of the one or more difference values stored in the storage and (ii) a latest difference value obtained when the switching condition is newly satisfied, is equal to or larger than a threshold.