As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
L ISLN VALVE SWITCH STUCK IN OPEN POSITION. AIRCRAFT BACK TO BLOCK AND GROUNDED BY MAINTENANCE. MAINTENANCE DEFERRED THE LEFT ISOLATION VALVE SWITCH IAW MWL 36-11-6. MAINTENANCE PROCEDURES COMPLIED WITH AND PLACARD INSTALLED. AIRCRAFT RETURNED TO SERVICE.