As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI OUT ON FLIGHT NR 3439, STANDBY GENERATOR STARTED TO RUN. A/C RETURNED TO GATE IN INVESTIGATE, STANDBY GEN INDICATED ON LINE, HOWEVER NO LOSS OF GREEN SYSTEM HYDRAULICS OCCURRED. STANDBY GEN CONTINUED TO RUN REGARDLESS OF SWITCH POSITION. MAINTENANCE REPLACED STANDBY GEN SELECTOR VALVE.