what i did was; put the car in reverse and pull the Ebrakes so it doesn't move or just have someone press on the brakes while it is in reverse, then listen to each sensor and if you hear a clicking / ticking sound, it's good. If not, that sensor has a problem.

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JL
'02 x5 3.0i
-Black on Titanium Silver (354)
-Sports Package
-Smoked LED Side Markers
-Chrome Gauge Trim Rings
-Retrofit AEs (Favorite Mod)
-Front & Rear OEM 4.6iS Diffuser
-Front & Rear Fender Flares
-K&N CAI