All models are also equipped with pyrotechnical front seat belt tensioners as part of the Supplemental Restraint System (SRS), to take up slack in the seat belt in a sudden frontal impact situation to reduce injury risk to front seat occupants. The tensioner is activated by a front impact over a predetermined force and lower impacts will not set off the system. When activated, the explosive gas in the tensioner pulls back and locks the seat belt preventing movement and holding the occupant securely in place; once activated the seat belt is permanently locked and requires replacement. Caution is recommended when working on the vehicle to prevent unintentional triggering of the system and it is recommended to work on the vehicle only by a Volvo dealer. For removal ign should be turned off and battery negative disconnected for no less than five minutes before removing the front seat, B-pillar trim and unplugging the wiring connector in the seat belt tensioner. The tensioner and seat belt reel can then be removed by unscrewing the retaining bolts. For rear seat belts (V70 models) access to buckles and floor anchorages requires tipping the seat cushion forward, and inertia reels are accessed by removing the rear seat back. The process consists of a number of steps, including headrests, plastic caps and screws removal and careful handling of joining rings, zip fasteners. For S80 models, the rear parcel shelf will need to be removed, and the battery disconnected before the tensioner wiring connector is unplugged. The retainers called seat belt anchorages and inertia reel retaining nut must be unbolted to remove the rear belts. Refitting is reversing the steps of removal and being sure that all components are securely tightened (a final check of the SRS warning light after re-connecting the battery).