An air bag deployment system includes circuitry for preventing the accidental deployment of the air bag during exposure of the system to an excessive external magnetic field. In a first exemplary embodiment of the system, the system includes an accelerometer comprised of first and second Hall effect magnetic devices which are associated with the impact detection circuitry of the deployment system. These are the sensors which would normally trigger deployment of the air bag system upon detection of an impact or acceleration of sufficient magnitude. A sensed magnetic field is used in determining the presence of an acceleration which is of sufficient magnitude to require deployment of the air bag in order to prevent injury to the occupants of a vehicle. The advanced system of the present invention further includes circuitry which senses large external magnetic fields. This sensing circuitry may be comprised of one or more additional Hall effect magnetic sensors which are located preferably outside the magnetic shield that contains the primary circuitry for triggering deployment of the air bag. When these external sensors identify the presence of an external magnetic field of sufficient magnitude, the sensors trigger circuitry which disables the air bag deployment circuit, thereby preventing inadvertent triggering of the deployment mechanism.