Image forming apparatus

An image forming apparatus comprising:

• an image forming unit configured to form an image;
• a transfer circuit configured to generate a transfer voltage, and supply the transfer voltage to transfer roller for transferring the image onto a sheet;
• a door provided to a mounting portion onto which the image forming unit is mounted;
• a sensor configured to detect an open/closed state of the door;
• an interlock switch configured to interrupt voltage to be supplied to the transfer circuit, in a case where the sensor detects an open state in which the door is opened;
• and
• a controller configured to:
• determine whether a failure has occurred or not based on a detection result of the sensor and a supplying/interrupting state of the voltage of the interlock switch;
• and
• determine, in a case where determining that the failure has occurred, which of the sensor and the interlock switch is failed based on a change in the detection result of the sensor and a change in a supplying state of the voltage of the interlock switch, which are caused by an opening/closing operation of the door performed after the occurrence of the failure is determined.