The fuse referred to in code 51 is internal to airbag diagnostic monitor and is not servicable, therefore new monitor required ($200 I believe). In the interim you have no air bag protection.
From another air bag thread I gather that you just bought this car, although you didn't mention whether private or dealer sale. In the jurisdiction where I am located cars are required to be certified as fit by a licenced mechanic before new ownership can be registered and car plated. I doubt a car displaying an air bag fault can be certified. Discussing vehicle history with seller might be helpful.
I'ts possible fault occurred because of prior air bag deployment so I suggest taking car to Ford dealer for inspection/diagnosis and an estimate for repairs. The following tech details from service manual may be of interest to readers.
AIR BAG DIAGNOSTIC MONITOR INTERNAL THERMAL FUSE - FUSE BLOWN DUE TO INTERMITTENT SHORT TO GROUND - Air bag code 51
CAUTION: Do not install a new air bag diagnostic monitor until the short has been located and corrected. If a short to ground has not been located and corrected, then the short to ground is intermittent and is not present at this time. Installing a new air bag diagnostic monitor with an intermittent short in the system will result in repeated blown air bag diagnostic monitors and repeat repairs.
REPLACE the air bag diagnostic monitor.
Thermal fuse not serviceable
Air bag diagnostic monitors can withstand several air bag deployments and do not need to be replaced after every deployment. Only replace the air bag diagnostic monitor if it is damaged.
Occasionally, immediately after an air bag deploys, the internal wiring of the air bag may become shorted to the metal housings of the air bag. This internal air bag short is detected by the air bag diagnostic monitor as a short to ground in the air bag deployment wiring. Since the air bag diagnostic monitor is still operating immediately after most deployments, the air bag diagnostic monitor will detect the shorted wiring and will flash diagnostic trouble code (DTC) 13 or DTC 14 and blow the internal thermal fuse. After the deployment, as the air bag cools off, the internal shorted wire may correct itself, therefore the short to ground will no longer exist and the air bag diagnostic monitor will flash DTC 51. If a vehicle with a deployed air bag is flashing DTC 51, inspect and replace all the damaged areas of the vehicle with crushed wiring, sensors, etc. If no damage is found, assume that the deployed air bag was the cause for the intermittent short and replace the air bag diagnostic monitor when the new air bag is installed.
