Purchasing a bypass plug and buying the right one
If you have read the guide below and diagnosed your car correctly as only having a passenger occupancy sensor mat fault then you can buy the bypass plug here.
BMW airbag light fix and reset
If you have an air bag light on your BMW as shown in the video then chances are it is the Passenger Occupancy Mat. Even so, as there are other airbag issues then it is recommended you get the error diagnosed first to ensure what the problem is. For example, the error shown on your iDrive etc can be vague and this is just a general message and may be misleading so again a proper diagnosis is recommended. The rest of the guide shows you how to identify the system in your car and how to fit a bypass plug.
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Black box with 2 wired (blue/white) plug
Identifying the system in your car
Note: Do not disconnect anything just yet!
To get better access, move the passenger seat fully forward and upward to give as much room as possible. Looking from the rear of the seat (not the front), check fully under the seat to locate the Occupancy Mat Black Box as shown in the picture. It will have 2 plugs attached and have similar writing on it to the blue writing above (albeit a different colour)
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Removing the original plug
Before removing the plug
NOTE: It is advised to disconnect the battery but most people don't do this. If you are only unplugging the black plug (highlighted by green arrow) then it shouldn't cause any issues if you do not have the key in the ignition. Be aware that if you unplug the wrong plug then it may cause another issue that may need resetting.
Disconnecting the plug
To get the plug out you need to push the plug retaining clip and then pull the plug out while keeping the retaining clip pushed in. The picture below shows where you need to push (green arrow) and push the same way as you were trying to push the arrow down (look at the bypass plug to get an idea of how the clip works). Support the black box and the tube bit that it's plugged into and while pushing the clip in, pull the plug off to remove it. You can now tuck this plug and wire away securely as it is now redundant.
Video of removal and fitting
Here is a link to a short video that shows how you unplug the original plug and insert the bypass plug. This is obviously simple due to the car being out of the seat but there is no need to take out the seat, this was only for better pictures and video
Fitting the bypass plug
Now the plug is out, fit the bypass plug. This is pretty straight forward but please support the back of the socket where the other blue/white wires come out. Put the bypass plug in and pinch the plug in with your forefinger and thumb on the blue/white wire end (or vice versa depending on which hand you use) and squeeze until you see it's clipped on. The place to squeeze is in between the 2 arrows in the pic below. Once clipped on pull gently to make sure it doesn't come off to check it's secure
Once everything is fitted securely you can then test the system. In certain cars such as some of the E46 models the error may not need resetting. If the airbag light is still on then it should simply need a reset to clear the error. One side effect is that seatbelt warning light may come on but there is a bypass guide for that also.