Errors related to the restraint system are a common issue for BMW, especially with cars made from 2000 onwards.
When there is an issue with seat belts, a message will pop up on the iDrive display and warn the driver.
Besides that, there will be an airbag warning light and a seatbelt warning light on the dashboard.
Depending on the cause, the dashboard warning lights might be on all the time or only when there is a person on the passenger seat.
- Seat Belt Tensioner: A tensioner within the seat belt buckle might have been activated. Although this happens during a collision, which is something that is unlikely to go unnoticed, there are other possible causes. Damaged wiring can cause a short-circuit situation and intended tensioner activation. A similar scenario is possible during the installation of seats. If the tensioner is activated, the seat belt error will be constantly present. You can test the tensioner with a voltmeter, checking the resistance, which should be zero. The activated tensioner cannot be repaired and needs to be replaced.
- Damaged Connection: Another likely cause is a damaged connector or broken wiring. The belt buckle has several wires, which go into a connector underneath the seat. Sometimes a foreign object, such as a bottle, can get stuck under the seat. This can damage the connector or wiring, especially when the seat goes back and forth.
- Corrosion: Moisture, dirt and other contaminants can enter the aforementioned connector, causing a poor connection. Carefully unplug the connector and check for dirt or water traces. If necessary, clean them out. After reassembly, make sure the connector is seated properly.
- Sensor: Lastly, there is a sensor within the belt buckle itself, which registers if the seatbelt is buckled. Although sensor failure as a result of intensive use is possible, it is not likely. On the other hand, spilled coffee or soda could easily end up inside the buckle. In combination with belt buckling, this can lead to sensor failure. To prevent this, always dry and clean off liquids accidentally spilled into the buckle.
Always take extra care when working around seatbelt pre-tensioners, as they contain an explosive charge.
To prevent accidental activation, disconnect the battery negative lead and wait at least 15 minutes before working on the airbag system.