As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
HOT AIR AND SMOKE ENTERED CABIN AND COCKPIT AREAS DURING TAXI OUT. RETURNED TO GATE. REMOVED AND REPLACED NR 1 AIR CYCLE MACHINE DUE TO SEVERE OIL LEAK, ALONG WITH DISCHARGE SENSOR AND BAG PER BOEING M/M. ALSO, REMOVED AND REPLACED NR 2 AIR CYCLE MACHINE AND BAG AS PRECAUTIONARY PER BOEING M/M.