A method and device detect printing defects on prints produced by an electrostatic printing device and, in response, place the electrostatic printing device into a diagnostic mode. While in the diagnostic mode, the method and device charge an image bearing surface of the electrostatic printing device to a uniform potential charge and disable all light emitting devices of the electrostatic printing device that could alter the uniform potential charge. Next, this method and device transfer marking material to the image bearing surface to create a test image and print at least one test patch by transferring the test image from the image bearing surface to a sheet of media. This method and device evaluate whether the test patch includes the printing defects and identify whether the light emitting devices of the electrostatic printing device are defective, based on whether the test patch includes printing defects.