Use f/16 to check for dirt on your image sensor

Is your camera sensor dirty?  Don't want to pull the lens, or maybe the dirt is so tiny you can't see it.  A quick way to check for dirt is to set the iris to f/16.  This will cause the dirt to cast a shadow on the sensor. 
If there is dirt, and you can't clean it.  Use a wide open iris.  The multi-angle light rays will obscure the dirt.