GETTING halves of two cars was not what Paulette Day expected when she bought a red 2004 Chevrolet Monte Carlo that year from a dealership near Detroit for $22,000. The car was used, but it was a “G.M. certified” car, meaning it had supposedly passed a rigorous inspection by the dealer. As General Motors says in its marketing material, buying a certified car means “the reliability of new and the affordability of used.”

Not in this case. Ms. Day said she became suspicious about the car after noticing the paint did not match. After a mechanic put the car on a lift and saw the welds, Ms. Day learned that it included pieces from the front of one Monte Carlo and the rear of another, both seriously damaged in crashes.

“I thought being certified, there are supposed to be so many checkpoints to make sure the car is safe,” she said. “I think they skipped over all of it. They would have had to notice that.”

Certified used cars have become popular over the last five years, favored by consumers worried about getting a lemon when they buy used. A guarantee from an automaker that the car checks out is peace of mind for which an increasing number of people are willing to pay extra, sometimes $2,000 or more. But some consumers are finding that certified does not protect them and some, like Ms. Day, are filing lawsuits.