yes every one of them on rear bumper
no but well I got bumper off I changed all of them , at the time it was forward think if one had packed up then it would not be long before the rest would so I changed all of them at the same time
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the most likely cause is the one is defective or not installed/connected correctly, which can bring the entire system offline
when the car is in reverse the sensors should be making a clicking noise when you put your ear close to each one. any that are not clicking are bad/disconnected
the only other possibility would be a module fault, but this would require proper computer diagnostics to verify/diagnose fully
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