As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHEN CONDUCTING THE SINGLE PACK SWITCH OVER FROM THE APU TO THE RIGHT 10TH STAGE AS PER THE PROCEDURE IN THE MEL, THE RIGHT 10TH STAGE BLEED VALVE WOULD NOT OPEN. RETURNED TO THE FIELD DUE TO LEFT A/C PACK BEING DEFERRED. C/W OPS CHECK OF 10TH STAGE BLEED VALVES, APU BLEED VALVES AND 10TH STAGE ISOLATION VALVES. ALL CHECK GOOD AT THIS TIME. LEFT PACK REPLACED.