How to check if the seat mat is 3-wired in Mercedes A W176, B W246, C W204, ML W166 and some others?

The seat occupancy sensor in most new Mercedes models may have 2, 3 or 4 wires:

  • white, red, brown (3-wired mat)
  • blue, brown, black (3-wired mat)
  • black, black (2-wired mat)
  • white, red, brown, green (4-wired mat in some american US version models with WSS system, rarely there may be different wires colors)
Below there are photos of the versions. The photos were taken under passenger side seat.

3-wired seat occupancy sensor
- brown, blue, black:

3-wired seat occupancy sensor
- red, white, brown:

2-wired seat occupancy sensor - black, black:

4-wired seat occupancy sensor - GLA USA

Restraint System Malfunction – what does it mean?

Due to numerous questions, we explain what to do if the on-board computer shows a message like this…:

or like this…:

or this…:

or this…:

or when the airbag / SRS warning light on the dashboard appears:

This means that one or more components that make up the SRS passive safety system have failed.

In all cases shown in the pictures above, computer diagnostics should be performed in order to find out what exactly is damaged in a car.
A good diagnostic computer will display codes with descriptions explaining which components of the safety system need to be fixed.

The information from the above pictures does not inform what exactly is damaged. You will find out about this from computer diagnostics.