As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW STATED OUTFLOW VALVE WOULD NOT CLOSE. AIR RETURN TO MSP. MAINTENANCE FOUND OUTFLOW VALVE MODE SWITCH OUT OF DETENTS. REPOSITIONED SWITCH AS REQUIRED, PERFORMED CONTROLLER SELF TEST, ALL CHECKED GOOD. PERFORMED CONTROLLER FUNCTIONAL TEST AMM 21-30-00 501, ALL CHECKED GOOD.