Airbag / SRS warning light after replacing seats

"I replaced the seat with a new one and now I'm having an airbag / srs light on. Do you have a device to turn it off?"

This is a problem that customers often contact us with.

For various reasons, the seat can be replaced. Sometimes the old one is destroyed, sometimes instead of a single one we prefer to have a double one (in minivans). It happens that users decide to completely remove one seat to have more space.

What happens when we disconnect the old seat

If we want to take out the old seat, we disconnect various components from the car installation – depending on the car’s equipment, that can be airbags, seat belt pretensioner, seat belt buckle, seat occupancy sensor, seat heating, electric seat control or some module.

What does the car computer see then?

Car’s control units recognize that particular parts have been disconnected. In case of disconnecting elements of the SRS system, the airbag control unit immediately displays an error in the form of an airbag / SRS indicator on the dashboard. Depending on the equipment of the car, the indicator may also appear on the middle screen, and sometimes also on the headliner (mainly in case of BMW from the USA / Canada).

When a particular component is disconnected, it will not work properly. The car installation treats it as an ordinary malfunction.

The SRS system is responsible for safety in the car and any inconsistencies are treated by control units as faults in the operating system.

What happens when we connect the new seat

If the original seat and the new seat are exactly the same and yet after connecting the new seat, the indicator lights up, it should be sufficient to remove the errors using diagnostic software.
In case this is not enough, make sure all parts have been properly connected. When you see nothing visually, computer diagnostics will show which component(s) causes the problem.

However, in most cases the new seat comes from a car with a different year of issue or different version. Sometimes this is the reason for the exchange, that it is somehow different. For this reason, it may have different equipment. In this case, removing fault codes without solving problems with components won’t hel – if the indicator goes off at all, it will most likely come back after a short time.

We explained that when disconnecting the old seat we disconnect the various elements from the car installation. Therefore, when connecting a new seat, all the elements that we disconnected should be connected. Sometimes, however, this is not possible – because the part(s) in the new seat does not occur or is different. In such case, you need to bypass such part(s) because car installation still contisues to demand it – the emulator of the part will help to bypass it.

The bottom of the seat with different connectors - Mercedes S W220

How do you know which components cause a problem?

If, for example, you have two plugins that are not connected to anything, then probably this two plugs make the warning light appear. If you do not know from which parts these plugs come from, it is best to perform computer diagnostics using the appropriate diagnoscope – it will show exactly which elements are causing the problem.

Once we know from diagnostics which elements require attention, and the reason for this is the fact that they are not connected to anything, we can use the emulator of the part that will simulate that the part is connected and works properly.

Solving a problem with the SRS/airbag warning light looks similar in different cars, even though the equipment is different.

Example I - Mercedes Benz

In Mercedes Vito / Viano, after removing the single seat (which is equipped with a seat occupancy sensor) and replacing it with a double seat (which is not equipped with a seat occupancy sensor), we will need a seat occupancy sensor (mat) emulator. After connecting it, you can remove the error of the faulty (in this case not connected) mat.

Is that enough?

Yes, if the seat mat is the only part that causes the light to be displayed.
No, if there are other fault codes in the airbag control unit which come from another components.
For example: if computer diagnostics also shows the seat belt tensioner fault code, then connecting only mat emulator is not enough – it will eliminate the problem of the seat mat, but the problem with the seat belt tensioner will remain. There is also a seat belt tensioner emulator – if for any reason we do not have a place to connect the plug from the tensioner, we can use the emulator. In this way, we repair / connect / replace / bypass any component that causes an error in the airbag control unit. Finally, we reset fault codes using the appropriate diagnostic software. Thanks to this, the control unit will receive relevant information from each component and stop displaying the light.

Example II - BMW

Another common and one of the more difficult examples is BMW E60, let’s assume the car is pre-facelift, with a metal key. The front passenger seat is replaced – the new seat, however, comes from the E60 facelift version. The seat occupancy sensors in the pre-facelift and facelift versions are different, so it will not be possible to connect the seat mat from the new seat to the installation of the car. It differs not only with the plug, but also with electronics.

Which emulator to choose – the one for the pre-facelift version or the one for the facelift?

The emulator must be connected to the car installation, not to the seat – it must be compatible with car installation then. Therefore, we need an emulator for the seat mat which was originally in the car – in this case the pre-facelift version.

Is the seat mat the only problem in this E60?
The answer is similar to the previous case. It depends on which elements were disconnected when removing the seat but were not connected to the installation after inserting a new seat. The seat mat is just one of the components. There are, among others, airbags, seat belt tensioner, seat belt buckle. If we are not sure – the easiest way is to perform computer diagnostics.

Replacing the seats does not always have to be troublesome. 
It is worth to carefully analyze all connectors.

Why is the SRS/airbag​ warning lit on the dashboard?

... and how to remove it?

If the SRS / airbag warning is on, it means that one or more components of the SRS system have failed.

The airbag system is controlled by itself – it checks the operation of all components of the system as soon as the ignition is switched on. The warning light comes on and goes out after about 4-6 seconds.
If the light does not go out, it indicates a malfunction in the safety system. It is then necessary to check the SRS system (e.g. in a service station) using a suitable diagnostic software.

For what? Isn't it easier to replace something right away?

Without good computer diagnostics, you don’t know which exactly elements of the SRS system are damaged and cause the warning light. And there are really many of such components which may cause the problem. Replacing or repairing various elements intuitively without carrying out diagnostics is the worst idea – it usually ends not only with the release of a fortune, the exchange of elements that work correctly, but also with the lack of knowledge of the construction and operation of the system may cause additional defects. Remember that sometimes the warning light can appear due to even the disconnection of one of the wires or contact impurities.

Click on the photo to enlarge
Click on the photo to enlarge

The picture on the left shows the airbag light “Passenger restraint system”. The picture on the right – message displayed on the on-board computer. None of the pictures shows what exactly is damaged in the car.
So what do the pictures show?
The information displayed both on the dashboard (in the form of a light) and on the on-board computer (in the form of a short message) only tell us that there is a fault in the SRS system – one or more components have failed. They do not say exactly what these components are and what is wrong with them.

SRS (additional passive safety system) consists of three groups:
• impact sensors
• airbag control unit
• actuators – including airbags, seat belt pretensioners, seat occupancy sensor, communication and automatic accident notification systems, battery disconnection systems.

To enable easier identification of the faults, in some cars the way of the warning light flashing has been programmed in the diagnostic module. For example, in many Mazda models, you can see that the light is flashing in a certain cycle. Flashing cycle 47 or 48 (4 flashes – break – 7 flashes) means a damaged seat occupancy sensor (mat) or a fault in the connection between the seat mat and the airbag control unit. However, it should be remembered that the boards decoding such a sign code are not identical for all types and models of cars, even the same manufacturer. In newer cars such solutions are no longer used.

After years of experience in the repair of safety systems, we state that Mercedes are one of the most difficult to diagnose cars. Cheap scanners usually do not give advice (they may incorrectly indicate damaged elements), it is worth going to a mechanic with a good diagnoscope, such as Star Diagnosis. Also the knowledge of the mechanic is not without significance – an unqualified mechanic using professional software can misdiagnose the error.

Some cars "hint" where to look for a fault.
Translation to English: "Malfunction in front of the right side, go to workshop"
The picture comes from a Mercedes E-Class W213.

It would seem that some things you can check yourself – even connections, plugs, wiring harnesses. If you decide to tinker with airbags, remember to disconnect the battery – even the smallest short circuit can activate the gas cushion and, consequently, seriously damage the body. It should also be remembered that improper handling with the car installation can lead to major faults. Often customers complained that when disconnecting the battery or disconnecting the connectors such components as battery safety terminal or airbag control unit were damaged or airbag control unit coding changed. Therefore, it is worth to leave repairing the SRS system to a qualified person.