As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DFW - CREW REPORTED LEFT CONSTANT SPEED DRIVE OIL PRESSURE LOW LIGHT FLICKERED ON AND OFF. CREW UNABLE TO START APU. EMERGENCY DECLARED FLIGHT RETURNED DFW LANDED WITHOUT INCIDENT. UPON DESCENT ABLE TO START APU. AIRCRAFT REMOVED FROM SERVICE. REPLACED CHARGE AND SCAVENGE FILTER ON LEFT ENGINE CONSTANT SPEED DRIVE. SYSTEM GROUND CHECK NORMAL OPERATION.