Code 12 - Loss Of Battery Power To Air Bag Firing Circuit: Description
WARNING: This page is about a different car, the 1995 Ford Pickup, 1995 Ford F-Super Duty, 1995 Ford Cab & Chassis, and 1995 Ford Bronco. However, it is still accessible from the selected car via links, so may be relevant.
Diagnostic monitor measures voltage at terminal No. 13 of diagnostic monitor connector. Reading should be battery voltage. If voltage drops to less than 9 volts, Code 12 will set.