As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PREFLIGHT CREW REPORTED, A BURNING RUBBER SMELL INSIDE THE AIRCRAFT. MAINTENANCE INSPECTED THE AIRCRAFT AND COULD NOT FIND ANY CAUSE OF THE SMELL. SUSPECTED CAUSE WAS THE EXTERNAL AIR CONDITIONING CART HOOKED UP TO THE AIRCRAFT. THE RECIRCULATION FAN WAS REPLACED DUE TO A SMELL FOUND ON THE AIRCRAFT OVERNIGHT INSPECTION.