In a method for measuring the tensile stress of a running web, force sensors (7) are connected in the form of a Wheatstone bridge (11). An amplifier (18) amplifies a diagonal voltage (17) of the Wheatstone bridge (11). In order to be able to detect whether at least one of the force sensors (7) is defective, the Wheatstone bridge (11) can be loaded using at least one resistor (26) by means of at least one switch (24, 25). Comparing the measured values, with loading, with the measured values, without loading, determines whether the force sensors (7) of the Wheatstone bridge (11) are functional. Otherwise, an active error signal (28) is output.