Ultrasound-based parking aid - park distance control (PDC)

Useful information and valuable tips on all aspects of the ultrasound-based parking aid.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their work. The information provided on this website is intended for use by suitably qualified personnel only.

What park distance control (PDC) really means: Basics

The parking assistant is a driver assistance system that supports drivers during manoeuvring and parking and warns them of obstacles in the immediate vicinity of the vehicle.


The traditional parking aid operates on the basis of ultrasound that is emitted using several sensors. If an obstacle is detected, the driver is informed of this either visually or acoustically. The more ultrasonic sensors that are installed and spread out across the width of the vehicle, the more accurate the measurement result will be. Depending on the vehicle or on the system, LED displays, graphic representations on the screen or purely acoustic signalling devices can be installed to provide the necessary information.

System designations of some vehicle manufacturers

Acoustic Parking System (APS) Audi
Park Distance Control (PDC) Volkswagen, Seat, Skoda, BMW, Toyota, Ford, Mazda, Range Rover, Volvo
Parking Assist Modules (PAM) Ford
Parktronic Mercedes Benz
Parking Assist System (PAS) Volvo

Ultrasound-based parking aid (PDC): Design and function

The following system descriptions and diagnostic information are illustrated using the example of a Golf V Plus. The manufacturer's designation for the purely acoustic parking aid is park distance control (PDC). Because four ultrasonic sensors are installed in the rear bumper, this is referred to as a 4-channel system. 

Parking aid function

The parking aid is activated as soon as the reverse gear is engaged. The ultrasonic sensors mounted in the bumper emit a package of ultrasonic pulses in combined transmit and receive mode. The signals reflected by an obstacle (echo) are picked up again by the ultrasonic sensors, amplified and then transmitted as digital information to the control unit.


An algorithm in the control unit calculates the distance to the obstacle based on the transit-time difference of the signals. When in receive mode, an ultrasonic sensor also receives signals from neighbouring ultrasonic sensors. The control unit can thus evaluate the signals from several sensors and consequently calculate the minimum distance. The acoustic distance warning system kicks in at a distance of 160cm. The frequency of the sound signal, i.e. the beeping, changes with decreasing distance to the obstacle. If the distance is less than 20cm, a continuous tone sounds.


By disengaging reverse gear or when exceeding a speed of 15km/h, the parking aid is switched off.

Park distance control - system components: Overview

The parking aid for the rear section of the vehicle consists of the following components:

Control unit

The PDC control unit controls each individual ultrasonic sensor and the sound signalling device.
The control unit determines the distance to the obstacle from data provided by the digital input signals.

Ultrasonic sensors

Each sensor has its own electronics and is connected to the control unit with a data line. All sensors have a common power supply.

Structure/design of the ultrasonic sensor:

  • Decoupling element
  • Diaphragm
  • Electrical connection
  • Sensor housing

Acoustic signalling device

The signalling device is controlled by the control unit and generates an audio frequency (approx. 1kHz) via piezoceramics together with a resonator.

Technical data

Supply voltage 9 -16 volt
Ultrasonic frequency 50kHz - 60kHz
Detection angle: vertical / horizontal  60° +/- 10° degrees 
Range  250cm
Temperature range -40 to 85°C

Important information on handling and functioning!

On vehicles with factory-fitted trailer coupling, the PDC system is not activated when a trailer is being towed.
In some cases, smaller objects, bushes, poles or posts may not be recognised and consequently not picked up by the system, which means that without human intervention damage could be caused to the vehicle.

Safety notice!

Drivers bear sole responsibility for parking and for all other manoeuvres!
The electronic parking aid is a driver assistance system, but it cannot replace the due care and attention provided by a human driver. And as a result of physical limitations, dead zones or blind spots can also occur during ultrasonic measurement and object detection if sound reflection is poor. For this reason, drivers always remain responsible for the assessing of obstructions.

Parking aid defective: Effects

The following consequences can occur if the parking aid fails:

  • Fault lamp lights up (system-dependent)
  • Error entry in the control unit

Causes of a defective parking aid: Causes of failure

The following causes can be responsible for the failure of the parking aid:

  • Power supply faulty
  • External damage to bumper or sensors
  • Defective ultrasonic sensor
  • Defective sound signalling device (beeper)
  • Defective control unit

Checking park distance control (PDC): Troubleshooting

The ultrasonic sensors or rather their system functioning is monitored by the relevant higher-level control unit. Any occurring errors are stored in the control unit's error memory and can be exchanged/updated by using a suitable diagnostic unit. With some vehicle models, a warning message can also be displayed on the instrument cluster in the event of a system error. 


However, before control unit diagnostics are undertaken, it is first advisable to carry out a visual inspection of the individual system components as part of initial troubleshooting activities.

Control unit diagnostics

As part of control unit diagnostics, it is possible to employ the help of various functions and vehicle information as and when needed.

Error code

In this function the error codes stored in the error memory can be read out. For all subsequent troubleshooting, general information regarding possible effects and causes is presented in the error code descriptions.

Reading parameters

This function enables current measured values from the control unit, such as brake booster pressure or brake pedal position, to be selected and shown.


With this function the volume of the parking aid's warning beep can be adjusted.

Circuit diagrams

System-specific circuit diagrams can be taken from vehicle information and used for troubleshooting purposes. Here the PIN assignment on the control unit plug or the cable colours can be read off and used for further testing. 


The respective test depth and variety of functions can be set out differently depending on the vehicle manufacturer and is dependent on the relevant system configuration of the control unit.

Park distance control - maintenance and repairs: Important information

Mounting instructions

During installation of the ultrasonic sensor, care must be taken to ensure that the decoupling element is correctly seated. If the installation is faulty, send and receive operations may be adversely affected.


The ultrasonic sensors in the bumpers must be kept clean and free of ice in order to ensure adequate functioning of the PDC system.
And so as to prevent any damage occurring, cleaning the sensors with a high-pressure cleaner should be avoided.

Painting of the parking aid sensor

In the majority of cases, the visible surfaces of the ultrasonic sensors have been painted in the vehicle's colour.
In such cases, the manufacturer stipulates vehicle-specific paintwork guidelines in order to avoid any impaired functioning. So Volkswagen, for example, specifies that the paint thickness on the front and side surface of the membrane must not exceed 125µm.
With regard to this matter, the current, vehicle-specific information provided by each individual vehicle manufacturer should be observed!

Repair notes
  • Repair work on electrical systems in the vehicle may only be carried out by trained specialist personnel.
  • Repair work that is carried out incorrectly can lead to system failure and to severe personal injuries.
  • Please observe both the safety  and also the installation instructions provided by each individual system or vehicle manufacturer for all repairs to electrical systems.

Installing and removing the parking aid: VIDEO

Replacing the parking aid

This video provides vital repair information about replacing the parking aid.

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