What was found
CABIN WAS FILLED WITH SMOKE PRIOR TO PUSHBACK WHILE SITTING AT GATE WITH APU RUNNING AND BOTH PACKS SELECTED ON. HAD A L PACK AUTOFAIL CAUTION MSG ALONG WITH L PACK FAULT. OBSERVED DUCT TEM FOR L PACK ABOVE 99 DEGREES C. L PACK FAULT STATUS MSG DUE TO MT023-B1-007373 CAUSED BY PACK OVERTEMP. SHUT DOWN LEFT PACK. VISUALLY INSPECTED L&R PACKS WITH NO VISUAL DAMAGE NOTED. WITH L PACK OFF SMELL WENT AWAY AND TEMP INSIDE COCKPIT REDUCED. LEFT PACK WAS FOUND TO BE LOCKED UP INTERNALLY. REMOVED AND REPLACED ACM IAW AMM 21-51-00 P/N OFF/ON GG670-95009-7 S/N OFF 01799 S/N ON 01317. OPS AND LEAKS CHECKS GOOD.