An active illumination three-dimensional, 3D, sensor system, comprising: an illumination source configured to emit light pulses to a monitored area; an image sensor comprising an array of pixels, wherein the image sensor is configured to measure a quantity of light received at a pixel of the array of pixels and generate electrical charge in proportion to the quantity of light; a data output component configured to convert the electrical charge to one or more digital data values representing the quantity of light received at the pixel; a distance determination component configured to determine a distance of a surface within the monitored scene based on analysis of the one or more digital data values; and an auxiliary illumination source mounted inside a housing of the 3D sensor system and oriented to emit auxiliary light pulses toward an inner surface of the housing; wherein the image sensor is further configured to perform, during a diagnostic test sequence, a measurement of a subset of the auxiliary light pulses reflected from the inner surface of the housing and received at the pixel as reflected auxiliary light pulses, the system further comprises an analysis component configured to generate diagnostic data for the pixel based on the measurement, the diagnostic data indicating a failure of the system, the pixel is a first pixel, the measurement of the subset of the auxiliary pulses comprises a first series of temporally sequential measurements of the subset of the auxiliary light pulses, the data output component is configured to convert results of the first series of temporally sequential measurements to a first set of data values, the image sensor is further configured to perform, during the diagnostic test sequence, a second series of temporally sequential measurements of another subset of the auxiliary light pulses reflected from the inner surface of the housing and received at a second pixel of the array of pixels, the second series of temporally sequential measurements occur simultaneously with the first series of temporally sequential measurements, the data output component is configured to convert results of the second series of temporally sequential measurements to a second set of data values, and the analysis component is configured to perform correlation analysis on the first set of data values and the second set of data values, and in response to determining, based on a result of the correlation analysis, that a correlation exists between the first set of data values and the second set of data values, disable the first pixel and the second pixel and instruct a user interface component to generate an error message. an illumination source configured to emit light pulses to a monitored area; an image sensor comprising an array of pixels, wherein the image sensor is configured to measure a quantity of light received at a pixel of the array of pixels and generate electrical charge in proportion to the quantity of light; a data output component configured to convert the electrical charge to one or more digital data values representing the quantity of light received at the pixel; a distance determination component configured to determine a distance of a surface within the monitored scene based on analysis of the one or more digital data values; and an auxiliary illumination source mounted inside a housing of the 3D sensor system and oriented to emit auxiliary light pulses toward an inner surface of the housing; wherein the image sensor is further configured to perform, during a diagnostic test sequence, a measurement of a subset of the auxiliary light pulses reflected from the inner surface of the housing and received at the pixel as reflected auxiliary light pulses, the system further comprises an analysis component configured to generate diagnostic data for the pixel based on the measurement, the diagnostic data indicating a failure of the system, the pixel is a first pixel, the measurement of the subset of the auxiliary pulses comprises a first series of temporally sequential measurements of the subset of the auxiliary light pulses, the data output component is configured to convert results of the first series of temporally sequential measurements to a first set of data values, the image sensor is further configured to perform, during the diagnostic test sequence, a second series of temporally sequential measurements of another subset of the auxiliary light pulses reflected from the inner surface of the housing and received at a second pixel of the array of pixels, the second series of temporally sequential measurements occur simultaneously with the first series of temporally sequential measurements, the data output component is configured to convert results of the second series of temporally sequential measurements to a second set of data values, and the analysis component is configured to perform correlation analysis on the first set of data values and the second set of data values, and in response to determining, based on a result of the correlation analysis, that a correlation exists between the first set of data values and the second set of data values, disable the first pixel and the second pixel and instruct a user interface component to generate an error message.