What was found
GEAR DISAGREE WARNING ON GEAR RETRACTION AFTER TAKEOFF, LEFT MAIN INDICATING RED. WENT AWAY WITH QRH PROCEDURE, GEAR REMAINED DOWN AND LOCKED ON LANDING AT SFO. (FLIGHT 5951 - FILED CLOSED)REFERENCED FAULT ISOLATION MANUAL 32-60-07, UPON TROUBLESHOOTING FOUND FAULT CODE S20 LGUPLK STORED IN PSEU. CLEANED DEBRIS FROM UPLOCK HOOK ASSEMBLY PROX SENSOR TARGET IN REFERENCE TO AMM 32-61-05. NO OTHER DEFECTS NOTED IN LANDING GEAR RETRACTION SYSTEM. RESTORED AIRCRAFT TO NORMAL CONFIGURATION, SYSTEM OPS CHECKS GOOD.