In this guide, we’ll break down the components of fault codes in heavy-duty trucks, focusing on SA, SPN, and FMI — key identifiers that help diagnose issues when your truck's check engine light comes on.

What Are Fault Codes?

Fault codes, also called Diagnostic Trouble Codes (DTCs), are generated through protocols like J1939. These codes help you understand the issue with your truck by providing specific information.

Breaking Down the SA, SPN, and FMI

When you encounter a fault code, you’ll see three key identifiers: SA, SPN, and FMI.

• SA (Source Address): This identifies the control module where the issue originates. Common examples include:
  • Engine Control Module (ECM) = SA 0
  • Aftertreatment Control Module (ACM) = SA 61
  • Body Control Module (BCM) = SA 33
• SPN (Suspect Parameter Number): This identifies the specific component or condition causing the fault. For example, SPN 3216 typically points to a problem with the Aftertreatment Intake NOx sensor.
• FMI (Failure Mode Indicator): This tells you the type of failure — such as circuit issues, component failures, or data communication errors. Common FMI codes include:
  • FMI 3, 4, 5, or 6: Circuit issues like open circuits or short circuits.
  • FMI 12: Component failure that requires replacement.
  • FMI 9: Communication failures with CAN (Controller Area Network) data transmission.

Common Fault Scenarios

• FMI 14: Requires special diagnostic instructions from the manufacturer.
• FMI 9 & 10: Indicate problems with data communication, such as delayed responses from components like VGT actuators or EGR valves.
• FMI 31: Often associated with non-electrical issues, such as an empty DEF tank or the truck being in derate mode.

By understanding SA, SPN, and FMI, you can identify the problem more accurately and decide whether a quick fix or a more detailed inspection is needed.