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BMW M328i, Airbag Light Fix

Yet another issue I wasn’t aware of until I had taken ownership of the car and driven away, was that the Airbag warning light was staying on too long, indicating an issue.

I’m not sure how or why it wasn’t lit on the test drive, maybe we had the car running long enough before I got in to drive it for the light to go away, but it was indicating there was an issue every other time I turned the key. The light would come on with the ignition, as it should, flash, but then stay lit for a good few minutes, before going out.

A lit Airbag light is a WOF fail, but for some reason the light does go out, it just takes ages.

Anyway, I fired up my old diagnostic laptop, launched INPA and checked for codes in the Airbag system. I had three. Two were related to voltage (low battery) and one was for “Sitzbelegungserkennung Beifahrer”.

Ah yup, that old chestnut. Translated to English, it means “Seat occupancy recognition front passenger”.

The occupancy sensor issue is a well known one in the E36 community. It is basically an electrical mat inside the base of the passenger seat, which detects when weight is in the seat and tells the SRS system there is someone sitting there, and in the event of an accident to fire the passenger airbag. If it doesn’t detect weight, it thinks no one is there, and wont fire the airbag.

The problem is that the mat can get worn or damaged over time, just from regular movement and pressure. Apparently kneeling on the seat base can damage them too (much more concentrated pressure). Once the mat fails, it will trigger the Airbag light and throw an error code.

The official way to fix it is to disassemble the seat, remove the old mat and fit a replacement. This is an excessive amount of work for little gain.

The unofficial way to do it, is to bypass the mat with a couple of resistors and a diode, which tricks the SRS system into thinking there is always someone in the seat, and to always fire the airbag in an accident. Some people DIY this with really ugly results, but a far tidier way to do it is to buy a proper bypass plug off the likes of eBay.

Mine cost about $11 plus shipping from a German seller, a hell of a lot cheaper than the $60+ they are asking for the exact same thing on Trademe.

Installation is easy. Disconnect the battery. Wait for a few minutes, then remove the four nuts/bolts that hold down the seat, and tilt it back to access the underside. This is where I discovered my occupancy sensor control unit under the seat wasn’t even plugged in… The wires were just floating around on the floor

That plug should be in the white socket on the bottom of this unit

Regardless, I chose to bypass the mat anyway. It was probably why it had been disconnected, in some misguided attempt to “fix” it.

Twist the unit sideways to release it from the seat base, unplug the black connector on the side, plug the bypass into the socket, and reattach the box to the seat base. Done. Easy.

Reinstall the seat, reconnect the battery, turn the ignition on and marvel at how the Airbag light is still lit.

I don’t know if it’s the same on all cars, but mine needed the codes to be cleared. I plugged my laptop back in, fired up my freshly updated (and now in English) INPA, and checked for codes. Sure enough there was one for the occupancy sensor. I cleared this, cycled the ignition, the light went straight out. I checked codes again, and nothing.

The live data from the system also shows that the “pressure sensor” is OK.

Great success. One more thing ticked off the list.

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