A control device with a valve adjusted by a control element is provided. A position sensor detects an actual position of the control element and an electro-pneumatic position controller generates a pneumatic control variable as a function of the actual and desired positions. The pneumatic control variable is supplied to the control drive via a magnetic valve activated by a supply voltage supplied by an instrumentation and control device. For carrying out a test of the magnetic valve, the magnetic valve is deactivated briefly. To initiate, monitor, and evaluate the test of the magnetic valve, the supply voltage is supplied to the magnetic valve via a controllable switch. The controllable switch is opened by a magnetic valve test signal generated by the instrumentation and control device. The magnetic valve test signal is supplied to the controllable switch via a limit switch which is opened when the control element reaches a specified position.