A known W126 quirk is that the speedometer's plastic gears strip and the odometer (or trip meter) stops advancing, which drives many 'true mileage unknown' discussions. Bidders scrutinize driving videos to confirm the odometer moves and cross-check Carfax mileage entries against claimed low totals.
Odometer plastic-gear failure and mileage verification
Mercedes-Benz S-Class (W126) · buying