Test image data is introduced into the image processing path of a marking device, at various points prior to the marking engine. Comparison of output of the marking engine from various sources of image data is made to diagnose imaging path component failure. Imaging data at various points in the image processing path is captured for analysis as part of component failure diagnosis. Various memory devices are used to both introduce test data at various points in the image processing path and capture data at various points of the image processing path.