One problem related to transmission not engage has been reported for the 2009 Ford Flex. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2009 Ford Flex based on all problems reported for the 2009 Flex.
Driving car, stopped at traffic light, or leaving places the transmission will not engage when put into drive. It gets stuck and won't move. This is very dangerous as I have almost been hit from behind when this happens. I have to put in park and try again and it sometimes shifts or makes a loud cluck and jerk. Happens off and on. Took into the Ford dealer couple of times and they said they could only determine what the problem is if it does it with them. Never does it when in the shop. The dealer acts as if they have no idea however I have read from several people that they have had the same problem with 2009 Ford Flex. I believe the transmission is the same as on the 2015 Ford transit vans because the same issue is happening with it. I have talked to my mechanic and they have heard of this problem too and told me to take the Flex into the dealer. They have found documentation online this transmission issue not shifting has happened several times. Most recently February 21, 2019. Seems to occurs when the vehicle is warmed up.