1. A video output checker which is configured to receive incoming video data for rendering an image on a display of a vehicle instrumentation cluster and to check whether a part of the image displayed in a fixed, predetermined area of the image is properly displayed by comparing the part of the image with a corresponding fixed, expected image, the fixed, expected image comprising an image of a telltale for communicating information regarding a state of the vehicle to an operator of the vehicle, the video data comprising pixel data which comprise, for each pixel, a set of colour component values for a given colour model, the video output checker configured to compare each colour component for a pixel in the fixed, predetermined area of the image with a corresponding test range of values and, if a component value falls outside the range, to measure a deviation, the video output checker further configured to sum deviations for the fixed, predetermined area to provide an error value for the fixed, predetermined area, and to compare the error value against a threshold number, and to output a message indicative of whether the image is properly displayed in dependence upon comparison of the error value against the threshold number.