As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON CLIMB OUT AIRCRAFT WOULD NOT PRESSURIZE. TRIED OTHER PRESSURE CONTROLLER AND MANUAL MODE, NO CHANGE. MAX DELTA P OBSERVED WAS 0.3 PSI. AIRCRAFT DID AN AIR RETURN TO ORD. FOUND PRESSURE CONTROLLER SWITCH/LIGHT LENS LOOSE IN PANEL. RESECURED. PRESSURE CHECK GOOD IAW AMM 21-31-00 PRESSURIZED TO 4.1 DELTA P NO DEFECTS NOTED.