As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAXI OUT, LT BLEED SHUTOFF VALVE FAIL AND LT PACK OVERHEAT MESSAGES APPEARED, SMOKE WAS APPARENT IN CABIN. AIRCRAFT RETURNED TO GATE. UPON INSPECTION, MAINTENANCE FOUND THAT THE CIRCUIT BREAKER FOR THE LT HX FAN HAD TRIPPED. MAINTENANCE RESET THE CIRCUIT BREAKER AND PERFORMED GROUND RUNS, OPERATIONAL CHECK GOOD.