A seat occupancy sensor is a component of the supplemental restraint system (SRS, or airbag system) in a vehicle. More specifically, the sensor is part of a subsystem called the Occupant Classification System (OCS) that senses the size and weight of an occupant in the front passenger seat in order to determine whether or not the airbag in that position should be armed or disabled. In some cases, the device also senses the position of the occupant in the seat and whether or not a seatbelt is being worn. Since the mid-two thousands, vehicle manufacturers have included seat occupancy sensors that can detect the presence of a person or an object in a seat.

When a vehicle is involved in a crash, special airbag sensors detect the impact and signal an electronic control module that, upon certain conditions, energizes the airbag inflators inside the passenger compartment. If no one is sitting in the passenger seat at the time, the passenger side airbag will not engage. If, however, a passenger is present, the airbag will inflate.

Unfortunately, if a passenger is small enough that the airbag might cause significant injury, as is possible with a child in the front seat, the seat occupancy sensor will disarm the airbag so as to prevent injury. Some sensors also allow the SRS to adjust the inflation of the airbag depending on the size and weight of the occupant. In many cases, the sensor is also able to distinguish between heavy packages and passengers. 

Seat occupancy sensors rely on different types of technology to measure occupancy. Some, for instance, rely on a silicone filled bladder, whereas others might use a device known as a strain gauge. When weight is present, an electrical signal is sent, sometimes by wire and sometimes wirelessly (depending on the vehicle make and model), to the electronic control module.