What was found
WHEN AIR TEMP SELECTOR KNOB IS ROTATED TO CHECK OUTPUT OF LEFT PACK, IT SHOWS OFF SCALE - 0 DEGREE. OUTPUT OF RT PACK SHOWS A NORMAL 18-22 DEGREE ALL ZONES ARE COMFORTABLE AND CONTROLLABLE. TRIM AIR IS ON AND PACKS ARE AUTO. GROUNDED - YES. REMOVED AND REPLACED PACK TEMP BULB ON LEFT PACK REF B737-800 AMM 21-51-60. CABIN TEMP. INDICATOR READS 19-21 DEGREES ON LEFT PACK. TEMP INDICATOR AND THERMOMETER READINGS AGREE. L PACK AND R PACK AGREE.