An apparatus for checking an error detection functionality of a data processing circuit, comprising an arithmetic logic unit, which provides an output datum based on an input datum, and an error detection circuit that executes the error detection functionality and detects an error based on the output datum during correct execution of the error detection functionality, and generates an error signal, if an error is present, which comprises a control circuit that passes the error signal through to an error signal output in a normal operating mode, and blocks the error signal in a checking mode, does not let the error signal pass to the error signal output, influences the arithmetic logic unit, the error detection circuit or the input datum such that the error detection circuit detects an error during correct execution of the error detection functionality, and, if no error signal is received in response to influencing, outputs an alarm signal indicating an incorrect execution of the error detection functionality.