Hi!
Take your X to any noiseless area,switch ignition ON and engage reverse gear.Listen and put the finger to
each sensor.You must heard and sense a 4Herz (four times per second) impulses from every sensor in front and rear of car. Replace sensor(s) which not transmitting ultrasonic signals. Another way is taking
your car to GT-1 or DIS computer to define which
sensor is faulty.Good luck!