As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMBOUT OF FLIGHT 2189 MKE-CLE THE CREW NOTE AN AMBER APU LIMIT EXCEEDANCE MESSAGE. THE SYSTEMS PAGE INDICATED LOW OIL PRESSURE. IT WAS DETERMINED THAT SINCE THE RIGHT DC GENERATOR WAS ALREADY ON MEL THAT THE AIRCRAFT WOULD HAVE TO RETURN TO MKE. THE CREW PERFORMED AN AIR TURNBACK TO MKE. MAINTENANCE REPLACED THE APU AND RETURNED THE AIRCRAFT TO SERVICE.