As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING OPENING ISOLATION VALVE WITH APU RUNNING AND ENGINES SHUTDOWN, A STRONG BURNING ORDOR OCCURED IN COCKPIT. ODOR DOES NOT PERSIST FOR MORE THAN A FEW SECONDS AND SEEMS TO BE ORIGINATING FROM THE COCKPIT A/C PACK. CYCLED LT AND RT PACK SWITCHES AND FOUND THAT THE ODOR OCCURS WHEN THE LT PACK SWITCH IS CYCLED. REMOVED AND REPLACED COCKPIT ACM. LEAK AND OPS CHECKED SATISFACTORY IAW MM 21-51-00.