All models equipped with pyrotechnic front seat belt tensioners as part of the Supplemental Restraint System (SRS) to give the seat belt instant take-up slack in the event of a sudden frontal impact, reducing the risk of injury to front seat occupants. The tensioner is activated by a frontal impact above some predetermined force, although lesser impacts will not activate the system. When triggered the explosive gas in the tensioner also retracts, this locks the seat belt to prevent the wearer from moving, and holds the occupant in position; once this has been activated the seat belt is permanently locked and must be replaced. Caution should be exercised when working on the car as it is easy to inadvertently trigger and be hurt, so it is recommended that such work be done by a Volvo dealer. For removal, the ignition should be turned off and the battery negative lead disconnected, and then wait for at least 5 minutes. The correct front seat and B-Pillar trim should be removed and the wiring plug disconnected prior to loosening the pyrotechnic tube clamp bolt and removes the seat belt inertia reel. For rear seat belts the same first steps apply, with special procedures for outer seat belts on S40 and V50 models with removal of the parcel shelf or folding down of the rear seat backrest. Accessing the centre seat belt inertia reels entails the removal of the seat cover, steps being taken to release the upholstery and armrests. Refitting includes reversing the steps for removing and making sure all components are properly fastened, and a final check of the airbag warning light after reconnecting the battery.