Why is my car's seat belt pre-tensioner warning light on?

The seat belt pre-tensioner warning light is stuck on. I've now taken my car back to the dealer twice, but they can't find the problem and say the car is fine to drive. Are they correct?

How can I replace my car's seat belts?

My Nissan X-Trail's seat belts need to be cleaned or replaced as they don't return back to their housing. Any tips on cleaning or replacement belts?

Citroen airbag and seat belt warning lights showing

Service light is coming on, seat belt pretensioner and air bags are mentioned as possible faults.