Ford Explorer 2011 Owner's Manual page 75

Climate Controls
Note: A cabin air filter must be installed at all times to prevent foreign
objects from entering the system. Running the system without a filter in
place could result in degradation or damage to the system.
For replacement intervals, refer to the Scheduled Maintenance chapter.
For more information regarding your filter, see your authorized dealer.
75
2011 Explorer (exp)
Owners Guide, 3rd Printing
USA (fus)