As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAXI OUT WHILE COMPLYING WITH RUDDER BIAS CHECK WITH THE ENGS AT POWER, CREW RECEIVED MASTER WARNING HORN. CREW RETURNED THROTTLES TO IDLE, NO CHANGE. CREW PULLED BREAKER & MASTER WARNING HORN STOPPED. CREW RETURNED FOR MX. MX POWERED ACFT NORMALLY & FOUND MASTER WARNING HORN DID NOT SOUND. ALL SYSTEMS OPERATING NORMALLY. ACFT RETURNED TO SERVICE.