The transporter sensor was a component aboard the 2270s refit USS Enterprise.
Just prior to the launch of the newly-refitted ship from spacedock, engineering technicians determined that a transporter sensor wasn't activated. This was attributed to faulty modules. Chief Engineer Montgomery Scott ordered Cleary to put in a new sensor. During this process, Commander Sonak and another crewmember were attempting to beam aboard. Cleary reported the transporters had reached the red line and attempted to contact the transporter room. When Scott ordered the room to disengage, Cleary reported that was it too late as the two individuals were beaming in. The inactivation of the sensor resulted in a malfunction of the system and the beaming failed with both individuals being killed. The defect was later corrected by Scott's engineering team. (Star Trek: The Motion Picture)