If the goods have been used for some time, or if there is undue delay in making the complaint, or if there is reason to believe that the goods have been accepted, the consumer's entitlement, at best, may be to a repair or to a partial refund. There are no hard and fast rules as each case has to be considered on its merits.
Where goods have been used for a time and subsequently prove faulty the right to reject them may be lost. In such circumstances a repair or replacement may be the proper solution. A repair should be permanent.