An open scattered light smoke detector for detecting smoke may include a light transmitter for emitting light, a light receiver spectrally matched to the light transmitter, and a control unit configured to repeatedly actuate the light transmitter, with a pulsed signal sequence, to emit corresponding light pulses, evaluate temporally a signal sequence received by the light receiver, and output a fire alarm if a received signal strength exceeds a minimum value for the smoke concentration. The control unit may be configured to switch the detector from a normal operating mode into a service mode if a phase angle between an emitted and received signal sequence, as determined on the detector side, increases by a minimum angular value which, in terms of the travel time, corresponds technically to an increase in the optical path length from the light transmitter to the light receiver of more than some predefined distance.