A method for monitoring the change in state of a valve in a start up circuit of a turbo-engine following a command to change the state of the valve includes measuring the air pressure under a fan cowl of the turbo-engine before the change state command, measuring the air pressure under the fan cowl after the change state command, comparing the measured values of the air pressure, and determining whether or not the valve has actually changed state. An aircraft turbo-engine includes a unit that monitors the change in the valve using such a method.