As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF AUTO PRESS INOP LIGHT INOP ILLUM, WENT TO MANUAL PRESS AND COULD NOT CLOSE OUTFLOW VALVE. OUTFLOW VALVE WOULD OPEN, BUT NOT CLOSE. AIR TURNBACK AIRCRAFT RETURNED TO LGA. FOUND AUTOPILOT SELECTOR KNOB JAMMED IN OUTFLOW VALVE CONTROL WHEEL IN PEDESTAL. REMOVED KNOB OUTFLOW VALVE, OPS NML IN MANUAL. RESET NR 1 AND NR 2 AUTO PRESS CONTROLLERS. AUTO NOW OPS NML. INSPECTED OUTFLOW VALVE FOUND NML.