I'm having issues with my A/C system. Even when the system is not running, I can feel air coming from the vents. I noticed the problem when I had my A/C on, switched to cabin circulate, but could still smell exhaust coming from the diesel truck in front of me. Turned off the system all together and still had air filtering into cabin from vents with exhaust smell. Never had this issue before, so wondering if there's a flapper issue.

Any advice would be appreciated.

 

Take out your heater/ac fan, reach up inside the plenum, if the valve flops around and moves easily with your hand the valve is broken. It is a major pain to replace as the dash has to come out. The valve is an integral part of the plenum. My truck is a 2010 too.