From the Ford service manual:
Diagnostic Trouble Code 51
The diagnostic monitor contains an internal thermal fuse that is not serviceable. The thermal fuse is controlled by the microprocessor inside the diagnostic monitor. The microprocessor inside the diagnostic monitor will blow the thermal fuse whenever a short on the deployment circuit occurs. The thermal fuse does not blow because of excessive current flowing through it. Do NOT attempt to jumper out the thermal fuse with a circuit breaker or any other type of fuse.
WARNING: Do not install a new diagnostic monitor until the short has been located and corrected. If a short to ground has not been located and corrected, then the the short to ground is intermittent and is not present at this time. Installing a new diagnostic monitor with an intermittent short in the system will result in repeat blown diagnostic monitors and repeat service.
The diagnostic monitor measures the voltages at the diagnostic monitor connector pins. When certain air bag wires are shorted to ground, the system may become susceptible to unwanted deployment of the air bag(s). The diagnostic monitor senses a short to ground on any of these circuits and helps prevent unwanted air bag deployment by blowing the diagnostic monitor thermal fuse. Blowing this fuse removes all power (battery and back up power) from the air bag deployment circuits. While the short to ground exists, the monitor will flash diagnostic trouble code 13 or 14, depending on where the short appears. If the short to ground is intermittent and temporarily corrects itself, the diagnostic monitor will flash code 51.
NOTE: If the short to ground returns, the higher priority codes 13 or 14 will be flashed instead of 51.
If the air bag indicator is flashing code 51 and a short to ground has not been serviced, this means that an intermittent short to ground exists in the air bag system. The diagnostic monitor should be replaced only after service of an intermittent short has been completed.
Some service tips for finding an intermittent short to ground are:
1. Consult OASIS (Restraint Sytems Service Code 104000) for up to date diagnostics and descriptions of wiring location concerns for the vehicle you are working on. OASIS is updated daily using concern descriptions from engineering and dealership service resources.
2. Inspect wiring harnesses in areas where they pass through or are located next to metal components (i.e. engine compartment bulkhead, body sheet metal, component mounting brackets, etc.)
Code 51 After Air Bag Deployment
NOTE: Diagnostic monitors can stand several air bad deployments and do not need to be replaced after every deployment. Only replace the diagnostic monitor if it is damaged.
Occasionally, after an air bag deploys, the internal wiring of the air bag(s) may become shorted to the metal housings of the air bag(s). The internal air bag short is detected by the diagnostic monitor as a short to ground in the air bag deployment wiring. Since the diagnostic monitor is still operating immediately after most deployments, the monitor will detect the shorted wiring and and will flash code 13 and blow the internal thermal fuse. After deployment, as the air bags cool off, the internal shorted wiring may correct itself, therefore the short to ground will no longer exist and the diagnostic monitor will flash code 51.
If a vehicle with a deployed air bag is flashing code 51, inspect and replace all the damaged areas of the vehicle that have 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 diagnostic monitor when the new air bag(s) are installed.
Code 51 Sequence of Events
1. Short to ground occurs on one or more of the circuits >
2. The diagnostic monitor recognizes the shorted wiring and flashes a code 13 or 14 (depending on where the short occured) >
3. The diagnostic monitor sends a signal to it's internal thermal fuse, causing it to blow >
4. The diagnostic monitor will continue to show the code 13 or 14 while the short to ground is present. If the short to ground goes away, a code 51 appears. >
5. The diagnostic monitor only flashes a code 51 when the thermal fuse is blown and the short to ground is not present. DO NOT replace the diagnostic monitor until the short to ground has been located and serviced.
