Method for operating a lighting device of a vehicle

The method involves operating a semiconductor light source i.e. LED, by pulse width modulated signals (I) with preset duty ratio between pulse duration (TE) and a period (T). The light source is applied during entire duration of diagnosis with pulse (TE0) of the pulse width modulated signals. The light source is operated during a diagnostic operation (32) deviated from a normal operation (34) of the light source. A period duration (T0) of the pulse width modulated signals during the diagnostic operation is selected to be greater than the period during the normal operation. Independent claims are also included for the following: (1) a control unit for a lighting device for operating a semiconductor light source (2) a lighting device comprising a semiconductor light source.