Airbag control unit with part number A4518201785

What does the airbag control unit A4518201785 do?

In the Smart, the airbag control unit A4518201785 belongs to the restraint system (SRS). It measures via the crash sensors what happens during an impact and decides within milliseconds which airbags and seatbelt pretensioners are triggered.

It also monitors the system constantly and shows faults via the airbag warning light in the instrument cluster.

After an accident it writes crash data to memory and locks itself.

Why does the control unit A4518201785 fail?

Crash data from an earlier accident is the most common cause, even if no airbag has deployed, and a normal diagnostic tool cannot erase it. Other causes include ageing components, cold solder joints, moisture and corrosion, and voltage spikes.

The fact that no airbag has deployed therefore does not mean that the control unit was left untouched.

The other causes concern the electronics and connections themselves: ageing affects the components, moisture affects the plugs, and a weak battery can generate voltage spikes.

How can you tell that the A4518201785 is defective?

A defect is indicated by the airbag warning light when it stays on permanently or intermittently. In addition there may be a fault message in the instrument cluster, no connection to the diagnostic tool, or a fault memory that cannot be cleared.

If these signs are present, the system does not work fully. In an emergency, airbags and seatbelt pretensioners may then fail to trigger.

The warning light is a signal, not a minor detail, so have it checked promptly.

Which vehicles is the A4518201785 fitted in?

The A4518201785 is fitted in these vehicles: Smart Fortwo C451 (2007-2015).

This list shows the assignment to the part number:

• Smart Fortwo C451 (2007-2015)

Check the part number on the label of the removed unit against this page. That only confirms that it is the right unit. Having the fault memory read out, on the other hand, brings the real clarification, because it shows whether the control unit is defective or whether a plug or cable is causing the fault.

For the same vehicles, we also repair control units with the part numbers A451 820 24 85, A4518201585/001, A4518202285, A4518202485/002, A4519011300Q02.

How does the repair of the A4518201785 work?

First have the fault memory read out and enclose the log. Then disconnect the battery, remove the control unit, create a repair order online and pack the unit well for shipping.

Airbag24 checks the unit on the test bench, renews defective components, erases the crash data and carries out a function test. Afterwards your own unit comes back to you. Testing, repairing and checking at Airbag24 usually takes 3–4 working days.

The order of the first steps is fixed for safety reasons. Our repairs come with 2 years' statutory liability for defects. The repaired control unit comes back with all vehicle data and is ready for use as soon as it is refitted — no coding or programming is required.

How does the control unit A4518201785 get to Airbag24?

You can send the unit as a parcel or book a UPS collection for an extra charge. After the repair, Airbag24 returns it to you by UPS.

Enclose the printed repair order, and make sure the control unit is packed shock-proof.