As of 2015, the top three most common problems with the Ford Flex are the gas gauge not working properly, the side-rear window shattering without being in an accident and the rotors rotting, according to CarComplaints.com. For the 2010 model, there are reports of complete engine failure before 20,000 miles.
The 2009 Ford Flex has the most complaints registered against it, including brake issues, transmission problems and drive train issues. Customers report the three main problems have happened before 40,000 miles and can cost anywhere from $500 to $1000 per issue to have fixed. Complaints about this car decrease considerably in model year 2011.