When an electronic system fault code is broadcast on a truck data bus, which field identifies the root cause of the problem?

When an electronic system fault code is broadcast on a truck data bus, which field identifies the root cause of the problem?

• A. PID
• B. MID
• C. FMI
• D. SID

Answer: (C)

In a truck’s diagnostic fault message, the parameter that failed is identified, but the field that pinpoints the reason for the failure is the Failure Mode Identifier. The FMI tells you how the fault occurred (for example, a short to ground, an open circuit, voltage out of range, etc.), which is the root cause of the problem. The parameter itself is indicated separately (the SPN), so knowing the FMI plus the SPN together gives you both what failed and why it failed. Other fields in the message indicate where the fault came from (the source ECU) rather than why it happened, so the FMI is the best indicator of the root cause.

In a truck’s diagnostic fault message, the parameter that failed is identified, but the field that pinpoints the reason for the failure is the Failure Mode Identifier. The FMI tells you how the fault occurred (for example, a short to ground, an open circuit, voltage out of range, etc.), which is the root cause of the problem. The parameter itself is indicated separately (the SPN), so knowing the FMI plus the SPN together gives you both what failed and why it failed. Other fields in the message indicate where the fault came from (the source ECU) rather than why it happened, so the FMI is the best indicator of the root cause.