VW PASSAT buzzer horn keeps ringing while it reverses

Yesterday, a 2007 VW PASSAT 2.8 V6 was taken to my garage because its buzzer horn kept blowing when it reversed. According to the car owner, the PASSAT was with 80,000 miles on it and never had this problem before.

After a brief inspection, I found 4 air ultrasonic ceramic transducers were damaged -- when I used VAS 5054A, a professional OBD2 tool for VW, AUDI, Bentley and Lamborghini, to trouble the car, I pulled four fault codes, which indicated problem with the 4 transducers. When a problem occurred to all of the 4 transducers, it was probably that the common part of them was damaged.

According to the circuit diagram of the air ultrasonic ceramic transducers, the first and third transducers were providing power. So I disassembled the tail bumper and tested the voltage of the sensor power supply and its bonding. The number was 11.10, which was within the normal range.

I put the tail bumper back, went on a test drive, and read data of the 4 transducers with VW diagnostic tool again. According to the result, the data of 2 transducers kept changing and the buzzer horn would blow when the auxiliary parking enforcement unit showed a safety distance of 36cm. I exchanged the 2 transducers, the problem stayed.

Thus it could be seen that the problem was with the auxiliary parking enforcement unit. I replaced the unit, and the buzzer horn could work normally then.