BMW 840i Backup Warning System Sensor Replacement Cost

Backup warning system sensors, also known as park assist sensors, park distance control (PDC) sensors, or simply park sensors, are safety devices, proximity sensors, that detect objects behind (or in the "proximity" of) a vehicle during parking. Some vehicles come factory equipped with backup warning sensors. Aftermarket systems are also available for retrofitting a vehicle with this feature.

When a car, truck, or SUV equipped with a backup warning system is shifted into reverse, the backup warning system scans the area behind the vehicle to locate objects that might be in the way. The backup warning sensors (park sensors) emit ultrasonic pulses that bounce off of an object and back to receptors in the sensor. When an object is detected behind the vehicle, a warning tone is heard audibly inside the passenger compartment to signal to the driver that the path is not clear. Some backup warning systems use electromagnetics instead of ultrasonic pulses. Some vehicles may also incorporate sensors that detect objects to the front or side of the vehicle.

A malfunctioning backup warning sensor may cause the switch that engages the backup warning system to remain lit (or display "off") even when the switch is turned on. If the vehicle is in reverse and there are objects in close proximity to the rear of the vehicle, but no warning tone is heard, one or more sensors might be bad. Some vehicles will display a dashboard warning light to alert the driver to a fault with the backup warning system.

Vehicles have been in operation for many years without backup warning sensors. A vehicle can certainly be driven without them. However, if a vehicle is equipped with a backup warning system and a driver assumes that the system is operational, injury or damage can occur. This safety system should be repaired as soon as possible to avoid accident or injury.

Backup warning sensors may wear out over time. The biggest problem with the sensors is probably dirt and grime that is allowed to build up. Simple cleaning of the sensors may extend their life or enable them to work properly. Of course, damage can result from a crash - even a minor one - that would render the sensors unuseable.

Diagnosing a faulty backup warning sensor is fairly simple. With the vehicle on level ground and the wheels chocked, and with the ignition in the "accessory" position, a technician will place the transmission in reverse to engage the backup warning system and then move to the rear of the vehicle near the sensors. Each of the sensors should make a faintly audible clicking sound when an object (a finger) is held a half inch or so in front of the sensor. The technician will test each sensor individually. Any sensor that does not emit a faint (but audible) clicking sound is not working. It is also necessary to make sure that the electrical connection to the sensor is operational. The technician may use a multimeter to test the connectors. A diagnostic scan to check for fault codes may be required.

The replacement procedure for a backup warning sensor on any vehicle will be similar: unplug the sensor, remove any retainer holding it in place, pull the sensor from its mount, install and connect the new sensor. Access to the sensor, however, may differ greatly from one vehicle to the next. On some vehicles, the sensor is easily accessed by reaching a hand inside the bumper. On others, the bumper assembly will need to be removed completely. This can be an extensive task on some vehicles. Once the new sensor(s) have been installed, the backup warning system may reset on its own, or it may need to be reset with a scan tool.

RepairPal recommends that a driver always perform a visual check around the perimeter of a vehicle prior to driving, especially backing up. It is also recommended that backup warning sensors be kept clean, free from grime or debris, to maintain functionality.

Always check behind the vehicle before backing up. Do not rely completely on the backup warning system. It should be noted that the sensors may not be able to detect small objects below the sensors, narrow objects (like a pole), or flat objects lying on the ground. Be aware of your surroundings and allow the backup warning system to assist. Also, if a diagnostic procedure is being performed and the vehicle is to be left in reverse while the sensors are inspected, it is essential that the vehicle be situated on level ground, preferably with the wheels chocked, to prevent the vehicle from rolling and causing damage or injury.

Anyone with intermediate DIY experience should be able to diagnose and replace the backup warning system sensors on a vehicle. It may be necessary to seek help if diagnostic trouble codes need to be read or cleared.