As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAXI OUT LT ENG GEN WOULD NOT PICK UP RT BUSS WHEN APU WAS SHUTDOWN FOR CROSSBLEED START. BITE CHECK OF BPCU FAULTED AUX CONTACTS/BPCU/GCU. ORDERED L C/B IAW FIM. REPLACED R531 IN P34 IAW WDM. PWR CKS GOOD MM. ETOPS CATEGORY 'E': PROBLEMS WITH SYS CRITICAL TO ETOPS.