It's registering an intermittent connection through your underseat connector plugs that fire seatbelt pretensioners in the event of an accident.
Many solutions I don't like a whole lot, as they involve lubricants or greases. The reality is, either the connecting wires have been strained to fracturing by seat movement while the plug is held too rigidly, or electrical contact in the plug's crimps is poor.
I also eschew direct soldering the connections without a plug; Murphy will intervene surely as night follows day and you'll be faced with disconnect/reconnect hassles.
- The repair is best performed with a low voltage soldering iron that has a fine, grounded tip and using light gauge electronic solder. Other soldering iron types present a risk of damage to the airbag controller.
I recommend standard (official) power-down procedure: Leave car with bonnet up and cabin unlocked for fifteen minutes or more, taking care not to use a remote transmitter near it in this time. This allows the bSI (in French, "CPH") to go into sleep mode and minimises risk of extraneous fault data being generated during a disconnect. Then without touching the doors or remote, disconnect the battery, positive first. Don't use a memory saver plugged into the lighter socket.
Wait another fifteen minutes, then unplug the brown underseat connectors. You may need to undo them from any clips or cable ties that hold their place under the seat frame. The time lapse here is to ensure storage capacitors in the airbag controller, have discharged sufficiently to negate any faint risk of false triggering.
Agreed that on many cars, this level of pedantry is not needed, but it is better to adopt a safe work method at all times than rue the one occasion you forgot!
Noting the order of wires, open the connector casings and slip out the connector pins/sockets. On the socket side, this requires a little tab (visible) to be teased up for each pin. On the male side, you need to release each pin by depressing a concealed tab - access better if you remove the small grounding contact strip inside the plug's mouth (it slides out with a little prying).
With all connector pins/sockets exposed, lay down a wad of folded newspaper (your drop sheet) and soft solder the crimped bit at mid-rear of each connector. It's readily seen; look for the bare wire ends. You'll know soldering is adequate when metal flows up the crimped portion towards the wire insulation. Allow a minute to cool, reassemble and hook up the battery.
Test by modestly straining/jiggling the connection with ignition on; as always keep your head away from the airbags. Note with some marques, dis/reconnecting the battery with an airbag fault prior, doesn't clear the fault light - you need a code reader and eraser.