After a minor collision: what to check even when it is "just a scratch"

A car park bump, a kerb strike, light contact in traffic. The airbags did not deploy, the car drives, visible damage is minimal. The logical conclusion is that nothing serious happened.

On a twenty-year-old car that conclusion would be right. On a modern one it is not always, and the reason is where the sensors live.

What sits in the first impact zone

A modern front bumper is not just plastic. Inside and behind it:

• The emergency braking radar — usually centrally behind the badge or in the lower section.
• Parking sensors around the perimeter.
• Surround view cameras.
• The crash sensor for the restraints system — it tells the module whether to deploy.
• Wiring to all of the above.

The rear bumper carries the same plus blind spot sensors.

That is why “just a scratch on the bumper” can mean a displaced radar worth fifteen hundred dollars.

The main trap: consequences are delayed

The typical timeline after a minor impact:

• Accident day — the car drives, everything works, you relax.
• A week later — some system’s warning lamp starts appearing.
• A month later — a persistent fault, and nobody connects it to that bump any more.

The reason is that a displaced sensor or a cracked connector does not fail instantly. It works with an error until vibration and moisture finish the job.

So the check belongs on the same day, while the link is obvious and while it is still an insurance matter.

What to check yourself in the first hours

1. The SRS lamp at key-on

The cheapest and most important check available. A healthy restraints module illuminates the lamp for 3–5 seconds and extinguishes it.

Never lights or stays on — the module recorded an event or was damaged. This is not “the airbags did not fire, so everything is fine”: the module may have logged a non-deployment event, and in some cases that requires servicing.

The recording mechanics are in the black box article.

2. Read codes across every system

Not just the engine. The other series matter here:

• B — restraints, for example B0001.
• C — chassis and wheel sensors: C0035–C0050.
• U — lost communication: U0121, U0151; mechanics in its own guide.

Save the freeze frame before clearing anything. It is both diagnostic information and a document for the insurer.

3. The module roll call

Who answers and who does not. A module that vanished from the list after an impact is the most alarming sign of all.

4. Parking sensors and cameras

Checked in a minute: select reverse, listen, look at the screen. A sensor beeping constantly or staying silent is displaced or has a cracked connector.

5. Geometry by eye

Even panel gaps, headlights sitting level, the car not pulling to one side in a straight line.

What requires a workshop

Three things you cannot check yourself.

Radar and camera calibration. Mandatory after any work on the bumper or windscreen. The dangerous scenario is not “the system does not work” but “the system works and is wrong”: braking for nothing, or missing an obstacle. Procedure in the ADAS calibration guide.

The restraints module’s state. Whether it holds a non-deployment record. Read with specialist equipment.

Suspension geometry. Even a light kerb strike shifts the alignment, and that costs tyres and handling.

Separately: why this matters for insurance

Minor collisions are often settled on the spot without paperwork — faster and less hassle.

The risk is that hidden damage surfaces a month later, when there is nothing left to prove the connection. A fifteen-hundred-dollar radar discovered five weeks on is yours to pay for.

So the minimum worth doing even in an informal settlement:

1. Photographs of the damage from several angles with the number plate visible.
2. A screenshot of the codes and freeze frame the same day. That is dated evidence.
3. The other party’s contact details.

Point 2 costs five minutes and saves you in exactly the scenario that is most likely.

What this means for a used car buyer

The mirror situation: what you check after your own accident is what a buyer will check on your car.

A concealed minor impact shows through the same signs: a silent SRS module, B and C series codes, a module missing from the roll call, an uncalibrated radar. The full procedure is in the rebuilt salvage guide and the inspection protocol.

In short

• The bumper houses the radar, parking sensors, and the crash sensor — a “scratch” can be expensive.
• Consequences surface weeks later, when the link to the accident is no longer obvious.
• The SRS lamp at key-on is the cheapest check and takes five seconds.
• Read B, C, and U series codes, not just P; save the freeze frame before clearing.
• Even in an informal settlement, take photos and a code screenshot the same day.

Download the app — codes across every system, the module roll call, and freeze frames are read with an ELM327 adapter over Bluetooth SPP on Android.