What was found
CABIN RATE ALERT ON CLIMB OUT. CYCLED CONTOLLERS WITH NO CHANGE. LEVELED OFF AT 11,000 THEN DESCENDED DOWN TO 9,000 WITH CABIN ALT ALERT. AIR TURN BACK TO KIND. FOUND MIAN CARGO DOOR VENT DOOR OPEN. FLIGHT CREW SAID THEY NEVER HAD A DOOR OPEN INDICATION. OPENED & CLOSED DOOR SEVERAL TIMES. LIGHT WAS OUT INITIALLY BUT INDICATION DID RETURN TO NORMAL. SUSPECT SWITCH IS INTERMITTENT. DOOR LATCH IS SERVICEABLE AND LATCHES SECURELY. CARGO DOOR UPR A ALERT - INOP. VERIFY DOOR IS CLOSED AND LOCKED PRIOR TO BLOCK-OUT.REMOVED AND REPLACED VENT DOOR WARNING SWITCH (S1-1346), REF SSM 52-34-0. TESTS NORMAL IAW AMM 52-70-00-5.