The purpose of a damper, or so-called 'shock absorber', is to introduce controlled friction into the suspension system and dissipate the incoming-to-vehicle energy due to external excitations, in order to give dynamic stability to the vehicle. As damping of the incoming-to-vehicle energy is crucial for road handling and passengers comfort and safety, proper shock absorber operation plays α critical role in the proper and safe vehicle operation .In shock absorber operation, it is possible to identify three distinct types of friction: dry solid friction; fluid viscous friction; fluid dynamic friction. In this work, a common twin-tube shock absorber of a typical passenger car was investigated. The shock absorber had operated for about 150000 kilometres in normal every-day drive conditions. In order to investigate the impact of the worn mechanical parts separately from the quality of the hydraulic oil, numerous tests were run for all three conditions of the specific shock absorber: in the state it was delivered (used), after replacing all mechanical parts, and finally after replacing the hydraulic oil. The results were discussed assuming the manufacturer's standards and also comparatively between the three shock absorber refurnishing conditions.