'''4SNS'''/ 1 / 40000000: '''T''' '''H''' 00 - 9.000
4SNS - System sensor
T - Temperature
H - Hard disk
As you can see yes the sensor within the HD is not active.
Did you replace the HD with a SSD or a HD? If not your system should still be covered under extended warranty. You'll need to talk with Apple.
If you swapped out your drive then the drive you put in does not comply with the newest SMART spec.