Possible Causes
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.
Code 51 will set if there is a short to ground in circuit No. 623, 624, 625, 614, 615, 617, or 619. A deployed air bag may also cause a Code 51 to set. When air bag deploys, internal wiring of air bag may become shorted to metal housing inside of air bag module.