Ford 2011 F-250 Owner's Manual page 211

For proper care of soiled safety belts, refer to Interior in the Cleaning
chapter.
WARNING: Failure to inspect and if necessary replace the
safety belt assembly or child restraint system under the above
conditions could result in severe personal injuries in the event of a
collision.
2011 F-250/350/450/550 (f23)
Owners Guide, 1st Printing
USA (fus)
Seating and Safety Restraints
211