An electronic device includes a first array of image pixels having inputs coupled to first selection tracks and outputs coupled to first output tracks, a second array of test pixels having inputs coupled to second selection tracks and outputs coupled to the first output tracks, and a third array of test pixels having inputs coupled to the first selection tracks and outputs coupled to second output tracks. A processor is coupled to receive output signals on the first and second output tracks. The output signals from the test pixels of the second and third arrays are fixed at one or the other of only two values in the absence of a defect. The output signals received by the processor over the first and second output tracks are processed to determine presence or absence of a defect.