GM/Duramax Allison 6 Spd Transmission Neutral Safety(PRNDL) Switch. When this sensor goes bad, you'll likely get a "shift range inhibited" message on the message center along with a flashing light under the gear or range you are attempting to shift to.
Additional info:
The vehicle is equipped with a column type shift selector. In addition to the column shifter, another component associated with the shift selector is the Neutral Start Back Up (NSBU) switch mounted on the selector shaft of the transmission internally. The NSBU switch transmits the gear selector position information to the TCM. The NSBU switch mounts inside the transmission housing, in the valve body, and detects the angular position of the shift selector shaft. This position is communicated to the TCM so that certain vehicle control functions can be coordinated with the position of the shift controls. The neutral signal output of the NSBU switch is typically used as confirmation that the transmission is in NEUTRAL or PARK before the engine starter is engaged.