Parking Sensors for Your Car: How They Work and How to Fit Them

Painted plastic bumpers are extremely expensive to repair, and one moment of distraction in an underground car park is enough to take one off. Parking sensors are, along with a reversing camera, the accessory that prevents the most trouble: they cost little, they can be fitted in a morning and they warn you with beeps and an LED display of the exact distance to the obstacle.

If your car predates these aids becoming standard, or if you have a basic trim level that went without them, here is everything you need to know before fitting a set.

How do parking sensors work?

Each sensor is a small ultrasonic emitter. It sends out a pulse that bounces off whatever is in front of it and measures how long it takes to come back: from that figure it works out the distance. The control box collects the readings from all four sensors and turns the result into two warnings:

• Audible: beeps that get closer together as you approach, until they become a continuous tone when the obstacle is very close.
• Visual: an LED display shows the distance in metres and which side the obstacle is on.

The system switches on by itself when you select reverse, because it draws power from the reversing light. It uses nothing at all while you drive forwards.

Sensors, camera or both

It is the most common question, and the short answer is that they do not compete, they complement each other:

• The camera shows you what is behind: a kerb, a bicycle, a child. But judging distances on screen is hard, and at night or in the rain it loses detail.
• The sensors show you nothing, but they give you the exact distance and detect low or very close obstacles that fall outside the camera's frame.

That is why modern cars have both. If you are only going to do one job, fit them together: they share the same power point at the reversing light and the dismantling work is the same. You can see the options in our parking camera collection.

What a 4-sensor kit includes

A pack of 4 parking sensors with LED display like the one we fit includes:

• 4 ultrasonic sensors with their seal and mounting clip.
• Control box with the wiring already made up and labelled connectors.
• LED display with distance readout and built-in buzzer.
• Hole saw of exactly the right diameter for drilling the bumper.
• Cable ties and double-sided tape to secure the wiring and the display.

The sensors usually come in black and can be painted to match the car at a body shop if you want them to go unnoticed.

How to fit the sensors step by step

1. Disconnect the negative battery terminal and keep a tape measure and masking tape to hand.
2. Mark the height: 50 to 60 cm from the ground is the recommendation, spaced symmetrically across the bumper. Too low and they will pick up the road surface and beep for no reason.
3. Remove the bumper if you can: it is the clean way to drill and to route the wiring out of sight.
4. Drill with the hole saw supplied, gently and perpendicular to the surface. File the edge and add a little rust-proof paint if the bumper is metal.
5. Fit the sensors the right way up: the arrow or mark on the body must face upwards.
6. Connect each sensor to its number on the control box, following the left-to-right order given in the manual.
7. Power the system: positive to the reversing light wire and negative to a body earth.
8. Route the LED display cable to the dashboard along the same path you would use for a camera and place it where you can see it without taking your eyes off the mirrors.
9. Test before closing up: reconnect the battery, select reverse and bring an obstacle close to each sensor to check that all four respond.

Common mistakes when fitting them

• Fitting them too low or at an angle: this is the number one cause of false alarms.
• Swapping the sensor order on the control box: the system works, but it tells you the wrong side.
• Taking power from the wrong wire: if the positive does not come from the reversing light, the sensors are either always live or never switch on.
• Not sealing the hole: moisture gets in and the sensor fails.
• Fitting them on a car with a towbar without allowing for it: the tow ball sits inside the detection area and it beeps constantly. The central sensors have to be moved.

Frequently asked questions

Do parking sensors pass the vehicle inspection?
Yes. They are a parking aid and do not affect the vehicle's type approval, as long as the bumper keeps its integrity and no lights or the number plate are covered.

Can they be fitted at the front too?
Yes, with a second kit powered from a point that is live with the ignition. In that case it is normal to add a switch so you can turn them off in stop-start traffic.

Do they work in snow or mud?
A dirty sensor loses sensitivity. A wipe with a damp cloth now and then is enough to get them reading properly again.

Can they be shown on the radio screen?
Universal kits use their own LED display. Some vehicles with factory sensors can show the information on the Android screen using a specific interface; ask us about your model.

Do I need a workshop?
Not necessarily, but drilling the bumper is the delicate part. If you would rather not, any auto-electrical workshop will do it quickly.

See the parking sensors

You will find the complete kit, with the four sensors, the control box and the LED display, on the product page for the pack of 4 parking sensors with LED display. And if you want to combine them with a camera, have a look at our guide on how to choose and install a reversing camera.