Vauxhall's used car brand Network Q is offering improved mechanical cover on all cars bought from January 1, 2001.
From now on the existing Network Q guarantee - 12 months and 15,000 miles - has been replaced by a new scheme with 12 months and unlimited mileage cover as part of the purchase price.
The existing six month guarantee on brakes, battery and clutch is to be increased to the full 12 months of the guarantee period.
These improvements come at the end of the year which has seen 112,000 Network Q cars sold.
"We are the first to offer an unlimited mileage, full wear and tear cover for clutches, brakes and battery," said Jonathan Leicester, Network Q brand manager.
"The decision to make these improvements was made with the full participation of our dealers. This means the Network Q guarantee improvements matches the confidence our dealers have in the quality of the cars they sell."
Changes have also been made to the age and mileage criteria for cars sold. Qualifying cars are now a maximum of five years old and no more than 60,000 miles - reduced from eight years/90,000 miles.