An image forming apparatus includes a printhead, an ink container, a sensor, a memory, and a processor. The printhead ejects droplets of ink to form an ink image. The ink container supplies ink to the printhead. The sensor detects an air leak in the ink container when an amount of air therewithin exceeds a given limit level. The memory records a count of air leaks detected by the sensor during a given counting cycle initiated at a given start time and completed following a lapse of a given period of time. The processor compares the detection count against a given threshold value upon completion of the counting cycle, and indicates a defect in the ink container when the detection count exceeds the given threshold value. A storage medium storing a computer-executable diagnostic program for use in such an image forming apparatus is also disclosed.