A pretensioner is a one time use deal like the bags. It pulls you back into the seat and releases the tension gently so your not flying into the airbag.
Pretensioners preemptively tighten the belt to prevent the occupant from jerking forward in a crash. Mercedes-Benz first introduced pretensioners on the 1981 S-Class. In the event of a crash, a pretensioner will tighten the belt almost instantaneously. This reduces the motion of the occupant in a violent crash. Like airbags, pretensioners are triggered by sensors in the car's body, and most pretensioners use explosively expanding gas to drive a piston that retracts the belt. Pretensioners also lower the risk of "submarining", which is when a passenger slides forward under a loosely worn seat belt. An alternative approach being looked at by major car companies is the CG-Lock technology whereby the occupant is held in position across the lap belt so do not come out of position in the event of a crash.
I'm no expert in this area, but if you just want to drive around I think your fine. You may throw a code. I'm not sure if the airbags deploy, the pretensions have to go off. As long as the belt still works I'm pretty sure your going to be ok.