The exit statuses of smartctl are defined by a bitmask but in decimal value. The eight different bits in the exit status have the following  meanings  for  ATA disks; some of these values may also be returned for SCSI disks.
Bit 0: Command line did not parse.
Bit 1: Device  open failed, device did not return an IDENTIFY DEVICE structure, or device is in a low-power mode (see '-n' option above).
Bit 2: Some SMART or other ATA command to the disk failed, or there was a checksum error in a SMART data  structure  (see '-b' option above).
Bit 3: SMART status check returned "DISK FAILING".
Bit 4: We found prefail Attributes <= threshold.
Bit 5: SMART  status  check returned "DISK OK" but we found that some (usage or prefail) Attributes have been <= threshold at some time in the past.
Bit 6: The device error log contains records of errors.
Bit 7: The device self-test log contains records of errors. [ATA only] Failed self-tests outdated by a newer successful extended self-test are ignored.