You need one flash killer for each side of the motorcycle (2 flash killers for one motorcycle). The flash killer ensures that the indicators on one side of the motorcycle are operating in the same frequency. Moreover, it ensures that there is no "defective bulb" error message in the speedometer.
Bulb Rear Light
Bulb 12V 21/5W BAY15d S25x47mm...
This powerful street-legal headlight bulb delivers up to...
E-homologated! Sold in pairs, amber bulbs included! ...