What was found
AFTER PUSHBACK, WE STARTED ENGINE #2, NORMAL START INCLUDING START SWITCH MOVING TO OFF AT 56%, BUT THEN START VALVE OPEN LIGHT REMAINED ILLUMINATED AND FLASHED. WE PERFORMED QRH PROCEDURE, SHUTDOWN ENGINE, STARTED #1 AND TAXIIED TO GATE. START VALVE OPEN LIGHT FOR ENGINE #2 REMAINED ILLUMINATED. GROUNDED - YES. PERFORMED EEC BITE CHK IAW B737-900 AMM 73-21-00. NO FAULT NOTED. R&R #2 ENG START VALVE PER AMM 80-11-03. OPS CHK FAILED. NO APU PSI RISE AND C/B P6-2 (C4) POPPING. SWAPPED IN ANOTHER START VALVE PER AMM 80-11-03. NO START VALVE OPEN LIGHT. R&R #2 ENG START SWITCH REF AMM 80-11-00. OPS TEST PASSED. PERFORMED POWERPLANT TEST 1 FOR #2 ENG START VALVE. TEST PASSED. OPS CHK GOOD. LEAK CHK GOOD REF AMM 71-00-00 AND AMM 80-11-03. START LIGHT SWITCH OPERATES NORMALLY.