As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT GENERATOR FAILED ON CLIMBOUT. STARTED APU AND AFTER POWERING BUS FOR 30 SECONDS APU HAD FAULT LIGHT AND SHUT DOWN. RETURNED TO ORD AND LANDED WITHOUT INCIDENT. REPLACED APU P2 SENSOR AND REPLACED LEFT ENGINE CONSTANT SPEED DRIVE GENERATOR.