What was found
STARTING #1 ENGINE AT THE END OF START SEQUENCE ENG 1 START VALVE OPEN LIGHT REMAINED ILLUMINATED - ALL THREE ENG LIGHTS FOR #1 ENG ILLUMINATED THEN FLASHED ON AND OFF - #1 START VALVE OPEN LIGHT STAYED ILLUMINATED REF QRH PROCEDURE - ISOLATION + PAC S/W WERE OFF - I FAILED TO PROPERLY APPLY STEP 6 - APU BLEED AIR SWITCH TO OFF - START VALVE OPEN LIGHT REMAINED ON - ENGINE SHUT DOWN. GROUNDED - YES. RESECURED ENGINE #1 START VALVE CONNECTOR MW0312. VERIFIED START VALVE IN CLOSE POSITION. PERFORMED DRY MOTOR AND START VALVE OPS NORMAL PERFORMED 2 NORMAL ENGINE STARTS WITH FLIGHT CREW, ALL ENGINE PARAMETERS NORMAL IAW B737-900 AMM TASK 71-00-00-800-808. A/C OK FOR SERVICE.