Understanding fault codes: what they mean and how to use them

When the check engine light illuminates on your truck's dashboard, it's time to pay attention to the diagnostic trouble codes (DTCs) that follow.

These codes are generated through a communication protocol, one of the most common being J1939, which is widely used in the trucking industry.

In this blog, we will break down what these fault codes mean and how you can use them to your advantage for better diagnostics and truck maintenance. 

What are DTCs? 

DTCs, or Diagnostic Trouble Codes, are identifiers that communicate issues with a vehicle’s systems. These codes provide vital information about the source of a problem, helping you pinpoint issues more accurately. The codes are structured with three key components: SA, SPN, and FMI.

• SA (Source Address): This indicates the computer system where the fault is originating. Common examples include: 

• ECM (Engine Control Module) = SA 0 

• ACM (Aftertreatment Control Module) = SA 61 

• BCM (Body Control Module) = SA 33 

• ABS (Antilock Braking System) = SA 11 

• TCM (Transmission Control Module) = SA 3 

If you see SA 0 in a code, it typically points to the engine control module. This helps you understand which system is reporting the fault. 

• SPN (Suspect Parameter Number): This identifies the specific component or condition triggering the fault. For example, SPN 3216 refers to an issue with the Aftertreatment Intake NOx sensor, pointing to a component failure. However, it’s also important to note that some faults relate to conditions rather than components, like an operating parameter falling outside the normal range. 

• FMI (Failure Mode Indicator): FMI provides more detailed information about the type of failure being reported, further narrowing down the issue. It can indicate problems like circuit breaks, component failures, communication errors, or operating conditions beyond set limits. For example, FMI 14 signals a special failure that requires specific instructions from the manufacturer to diagnose accurately. 

Common FMI Codes and what they mean 

When diagnosing faults, understanding the different FMI codes is crucial. Some of the most common FMI codes include: 

• FMI 3, 4, 5, 6: These indicate circuit issues, such as an open circuit, short to ground, or other electrical failures. 

• Open Circuit: A broken wire or disconnected part, preventing the circuit from completing. 

• Closed Circuit: Something that should be open is now closed, leading to improper operation. 

• Short Circuit: A grounding issue, often caused by internal component failure. 

By understanding these fault codes and their meanings, you can more quickly diagnose and address problems with your truck’s systems, ensuring smoother operations and preventing unexpected breakdowns. 

Diagnostic trouble codes (DTCs) may seem complex at first glance, but once you break them down, they offer valuable insights into what’s happening with your truck. By understanding the components of the code SA, SPN, and FMI, you can easily identify and address issues, saving time and money on repairs.

With the right tools, like OTR Diagnostics, you can have these fault codes at your fingertips and stay ahead of potential issues.