Express warranties are present in nearly all car sales. Under Minnesota law, an express warranty is any statement of fact or promise about the vehicle that the buyer makes to the seller. It doesn't matter if the buyer doesn't intend to create a warranty--if statement or promise is part of the basis of the bargain, it creates an express warranty. Although many express warranties are in writing, verbal representations can be express warranties too. Express warranties can't be waived by selling the vehicle "as-is." Probably the most common type of express warranty in a used car sale is the dealer's statements about the vehicle's condition or characteristics. To be an express warranty, the statement has to be specific (ie. "this vehicle doesn't need any major repairs') and has to be more simply sales puffery (ie. "this is a great car").
Another type of express warranty is when the seller promises to make repairs after the sale. It is common in many used car transactions for the seller to tell the buyer that she can bring the vehicle back if she has any problems. This is probably enough to create an enforceable express warranty.
If the seller breaches an express warranty, the buyer is entitled to sue for damages. This could included the diminished value of a vehicle that wasn't as good as promised or perhaps the cost of repairs that the seller promised to make.