Even if used car dealers or franchised dealers know of a recall, there is no requirement that they tell prospective buyers or make the repairs before a sale, said the Government Accountability Office study released Wednesday
Nor does the National Highway Traffic Safety Administration, which oversees auto safety, have the power to order dealers to make the repairs or to warn buyers.
The safety administration also lacks the authority to require automakers to inform used car dealers of recalls.
The situation "could pose a significant risk to the safety of millions of vehicle drivers and might have a negative impact on recall completion rates," the study said.
"Many consumers may be unknowingly putting their lives at risk by purchasing a defective vehicle," it said.
More than 35 million used cars were sold in 2009, including 11 million by used car dealers.
Used car dealers told investigators that part of the problem is that there is no database that they can check using a vehicle identification number -- which all cars and trucks have -- to see if there are recall-related repairs for that vehicle that haven't been made, the report said.
NHTSA estimates that average of 70 percent of repairs are made within 18 months of a recall. However, investigators said the average from 2000 to 2008 was 65 percent. The average also varied substantially from year to year.
It also varies significantly by manufacturer. Some manufacturers had a better than 90 percent average repair record for recalled vehicles, but at least one manufacturer averaged only 23 percent fixed and another 53 percent.
The National Automobile Dealers Association said in a statement that its goal is to have 100 percent of recalled vehicles repaired.
It urged used car owners to check with a franchise dealer who can search the manufacturer's database to see if all recall repairs have been made.
Wade Newton, a spokesman for the Alliance of Automobile Manufacturers, said in a statement that consumers can check directly with automakers or search the safety administration's Web site, Safecar.gov, to see whether there is a pending recall for a particular model.
However, since the government Web site isn't searchable by VIN number, it's impossible for used car owners to tell whether their particular vehicle has been repaired.
The study recommends the safety administration make its vehicle recall database searchable by VIN number and ask Congress to give it the authority to ensure car buyers are notified of recalls prior to sales.