As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FOR FLIGHT 4258, THE CREW REPORTED A STRONG ELECTRICAL BURNING ODOR IN THE COCKPIT AT THE GATE. UNABLE TO DETERMINE THE SOURCE. THE ODOR DID NOT DISSIPATE UNTIL AFTER COMPLETELY DE-POWERING AIRCRAFT. ON FURTHER INVESTIGATION, FOUND ELECTRICAL BURNING ODOR TO DISSIPATE ONCE PACK 1 AND COCKPIT RECIRCULATION FAN WERE SHUTDOWN. REMOVED AND REPLACED THE NR 1 RECIRCULATION FAN IAW AMM 21-24-01. OPS CHECKED GOOD. AIRCRAFT RETURNED TO SERVICE.