Ultrasonic Parking Sensor

They are the small round sensors in bumpers. They work at short range. Some cars now use cameras instead. Most cars have four to six sensors per bumper, which also feed park assist systems. A faulty sensor usually shows a warning or a constant tone, and the system may switch off completely. Sensors can be tested by listening for a faint clicking sound or with a scan tool that shows each sensor's readings.

The sensor emits sound pulses and measures the echo time.

Dirt, ice and paint can cause false warnings. Sensors must be painted carefully after repairs.