As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF, COULD NOT PRESSURIZE AIRCRAFT. FUEL WAS DUMPED AND THE AIRCRAFT RETURNED TO DTW AND LANDED WITHOUT INCIDENT. FOUND OUTFLOW VALVES VERY DIRTY. CLEANED OUTFLOW VALVES AND INSPECTED PLUMBING TO CONTROLLER. PERFORMED PRESSURIZATION TEST, CHECKED OK.