What was found
DURING CLIMB, RED MCD LIGHT ALONG WITH AMBER VENT DOOR NOT CLOSED ILLUM. ALONG WITH UNSCHEDULED CLIMB IN CABIN ALT. ACC. AERONAUTICAL MCD WARNING ILLUM. IN FLT. AFTER RAPID DEPRESS. CHECKLIST COMPLETED LAND AT NEAREST SUITABLE AIRPORT.CORRECTIVE ACTION:DUPLICATED MALFUNCTION ON GROUND AND FOUND VENT DOOR SWITCH ITNERMITTENTLY DEFECTIVE ALLOWING THE VENT DOOR NOT TO CLOSE FULLY. REPLACED VENT DOOR SWITCH AND SYS CHECKED NORMAL WITH A/C PRESSURIZED.