51: Air Bag Diagnostic Monitor Internal Thermal Fuse Blown

Electrical Schematic:

Normal Operation

NOTE: The air bag diagnostic monitor contains an internal thermal fuse that is not serviceable. The thermal fuse is controlled by a computer inside the air bag diagnostic monitor. The computer will blow the thermal fuse whenever a short on the deployment circuits 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.

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 blown air bag diagnostic monitors and repeat repairs.

The air bag diagnostic monitor measures the voltages at the diagnostic monitor connector pins. When certain air bag deployment wires are shorted to ground (heavy lines illustrated in the schematic below), the system may become susceptible to unwanted deployment of the air bag(s). The air bag 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 backup power) from the air bag deployment circuits. While the short to ground exists, the air bag diagnostic monitor will flash code 13. If the short to ground is intermittent and temporarily corrects itself, the air bag diagnostic monitor will flash code 51.

NOTE: If the short to ground returns, the higher priority code 13 will flash 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 supplemental air bag restraint system. The air bag diagnostic monitor should be replaced only after repairs to the intermittent short have been completed.

Some service tips for finding an intermittent short to ground are:
1. Consult OASIS (Restraint Systems Service Code 104000) for up-to-date diagnostics and descriptions of wiring concern locations for the vehicle (VIN number) you are working on. OASIS is updated daily using concern descriptions from engineering and Dealership Service sources.
2. Inspect wiring and 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
Occasionally, immediately after an air bag deploys, the internal wiring of the driver side air bag module may become shorted to the metal housings of the driver side air bag module. This internal short is detected by the air bag diagnostic monitor as 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 code 13 and blow the internal thermal fuse. After the deployment, as the air bag cools off, the internal shorted wiring may correct itself, therefore the short to ground will no longer exist and the air bag 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 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 driver side air bag module is installed.

NOTE: 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.

Code 51 Sequence of Events
1. Short to ground occurs on one or more of the circuits shown in heavy lines in the schematic.
2. The air bag diagnostic monitor recognizes the shorted wiring and flashes out a diagnostic trouble code 13 and sends a signal to the internal thermal fuse, causing it to blow.
3. The air bag diagnostic monitor will continue to flash code 13 while the short to ground is present. If the short to ground goes away, a diagnostic trouble code 51 appears.
4. The air bag 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 air bag diagnostic monitor until the short to ground has been located and serviced. Consult OASIS for shorted wiring information.