The sensors make a clicking noise when they are working. Put your ear up to the bumper and if one doesn't make the noise it is bad or the connection is loose.
2014 Carrera 2S
- White/2 Tone Espresso/Cognac - Aerokit - PSE - PDCC - SPASM - PDK - Sport Chrono - 20" wheels w/OEM Spacers - Glass Roof - Leather Dash - way too much more to list
2013 E92 M3
- LeMans Blue/Black Novillo - Comp Package - MDCT - CW - PP - EPS - CF Roof w/OEM CF Mirrors, Spoiler, Interior Trim - Full IND package - Muffler Delete
2013 F12 M6
- Alpine White/Black Merino - Executive - LEDs - B&O - DA - Lots more