As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DEN - ON FLIGHT 4177, DURING CLIMB, NR 1 ENGINE FAN VIBRATIONS WERE IN THE YELLOW ARC, WITH POWER REDUCTION, THE READING WAS 2.1. AIRCRAFT RETURNED TO THE FIELD AND ACCOMPLISHED A UNEVENTFUL LANDING. MAINTENANCE REPLACED THE N1 FAN BLADES. OPERATIONAL CHECKS GOOD. AIRCRAFT RETURNED TO SERVICE.