An elevator system drive unit moves an elevator car in an elevator shaft to at least two shaft access doors under control of a control unit. The car does not move or moves only to a limited extent if an individual is in the shaft. A monitoring unit and sensor (switching contact) detect changes in state in at least one of the doors using a sequence of pulses monitoring signal. The monitoring unit has a battery and can be switched to an autonomous mode when the elevator system is entirely or partially disabled. The monitoring unit, in the autonomous mode, records state data from the sensor and is connected to a safeguard unit that reads and evaluates the recorded state data, and prevents the elevator system from being put into the normal mode of operation if a change in the state of one of the monitored doors has been detected.