Backup Cameras and Parking Sensors

What it does

A backup camera shows the area behind the car on the dashboard screen when you select reverse. New passenger vehicles sold in the United States have been required to have one since May 2018. Many add guide lines: fixed lines mark distances behind the bumper, and on some cars a second set bends as you turn the wheel to show where you will go.

Parking sensors beep as you near an obstacle, the beeps speeding up as the gap closes and turning into one steady tone when you are very close. Surround view stitches several cameras into a picture of the car from above.

How it senses

The camera sits near the rear plate or tailgate handle and uses a wide-angle lens, which makes objects look further away than they are. Parking sensors are the small round discs in the bumpers. They send out pulses of sound too high to hear and time the echo.

What sets it off

Selecting reverse switches on the camera and the rear sensors. Front sensors usually wake below a low speed or at the press of a button. Some cars also brake automatically if they sense you are about to reverse into something.

Where it stops

• Rain drops, road salt or dust on the lens blur the picture.
• Thin poles, chains, low curbs and soft objects that absorb sound, such as snow banks, may not give a clear echo.
• Anything below or above the sensors’ field, including a small child sitting down, may not register.
• Heavy rain, a car wash or an aftermarket bumper part can cause false beeping.
• The camera cannot see what is beside the car, which is where people step out from.

Check for your car

• What each color of guide line means. Often red, yellow and green bands mark near, middle and far distances, but not always.
• Whether the lines move with the steering.
• How to wake the front sensors, and whether you can mute the tone.
• Whether your car can brake for you when reversing.

Before reversing, a walk around the car and a look in the mirrors and over your shoulder still catch what the screen leaves out.