Cold air out passenger vent with heater on. Found blend door loose on that side. on 2009 Dodge Journey

Replaced air box but problem continues. No codes. System is a manual single heat system.

3 answers 2 comments
I just had same problem...was told by autodealer to turn on heat and let cold temp setting cycle through...then shutt off and star up again.shud fix....so i defrosted for 5 mins at high heat.then synced heat to low woith high fan for another 5 mins.shut off vehicle.started vehicle and slowly increased synced heat...IT WORKED!
Could you explain this process again?
I replaced the Blend door actuator to na avail. It was a plugged Heater core. A big job requireing the dealer. 10 hours of labor to remove the dash and replace the core. $1800. Never heard of a core cgoing bad in 6 years.
door still loose? maybe incorrect installation
No door working and seals when out of the vehicle. Original door was cracked along shaft. new doors work in tandem manually and no movement on shaft.
Thank you for your response.