An image forming apparatus uses a plurality of image forming units having photosensitive drums and that are arranged rotatable in a vertical plane such that the image forming units are shifted sequentially to an image forming position. An air box is disposed in the engine compartment which includes a first air path and a second air path. The user interface includes a display screen configured for displaying graphical representations of the electrodes. When an alarm condition is detected, the display renders the alert screen in place of the home screen, the alert screen displaying information corresponding to the detected alarm condition.