Shortcomings
Since the system relies on the reflection of sound waves, it may not detect some items that are not flat or large enough to reflect sound, for example a narrow pole or a longitudinal object pointed directly at the vehicle or near an object.
Some objects such as skips (UK) or dumpsters (USA) may have flat surfaces that are angled from the vertical. These objects can behave in "stealth mode" by deflecting the return sound waves away from the sensors causing them not to be detected.
Ultrasonic sensors, being exposed to the elements and not being solid-state devices, frequently do not last the life of the vehicle and may require replacement after a number of years. Most PDC systems have a fail-safe mode which detects a malfunction and alerts the driver that the system is not working and that he/she should not rely on it when reversing. Typically this is done with a long continuous tone as soon as reverse gear is engaged.
The ease of replacing sensors varies; on some vehicles the rear fender (bumper) needs to be removed and the sensors may also require spray painting to match the rest of the car, making this an expensive proposition. On other cars the sensors are mounted on a separate piece of black trim which can be easily removed for maintenance.
Although the sensors are ultrasonic and the majority of their output is beyond the range of human hearing, it is possible to detect a completely failed unit by putting the vehicle into reverse gear (with the ignition on but, for safety, without starting the engine) and listening closely to each of them. A working sensor makes a rapid clicking sound; a sensor that does not click, or is noticeably quieter than the others, is likely to be the cause of a malfunctioning PDC system.
Read more about this topic: Parking Sensors
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