I just checked apogee website and they had a product announcement stating they discovered some defects and wrong readings in particular serial numbers of the MQ and SQ sensors
MQ-500 S/N 1001-1457
MQ-510 S/N 1001-1109
SQ-500 S/N 1001-1227
SQ-520 S/N 1001-1359
SQ-514 S/N 1001-1002