SPN 625 FMI 17 – No ACM2 Communication - Pre-Warning Fault Code (Symptoms, Causes & Fix)

Mar 13, 2026

The SPN 625 FMI 17 fault code indicates a communication issue between the Engine Control Module (ECM) and the Aftertreatment Control Module (ACM2). This diesel engine fault code suggests that the ECM is not receiving the expected data from the ACM2, which is critical for managing the aftertreatment system effectively. As a pre-warning, it alerts technicians to potential communication disruptions that could affect emissions control.

What SPN 625 FMI 17 Means

The SPN 625 FMI 17 fault code is a diagnostic trouble code that signifies a lack of communication with the ACM2. The ECM relies on data from the ACM2 to monitor and control the aftertreatment system, which includes components like the Diesel Particulate Filter (DPF) and Selective Catalytic Reduction (SCR) system. When this communication is interrupted, it can lead to improper aftertreatment operation, potentially increasing emissions and affecting engine performance.

Common Causes of This Fault Code

Faulty ACM2 module
Damaged or corroded wiring harness
Loose or damaged connectors
CAN bus communication issues
Faulty terminating resistor

Symptoms Drivers May Notice

When the SPN 625 FMI 17 fault code is active, drivers may notice a few symptoms. These can include the Check Engine Light illuminating, reduced engine performance, or even the engine entering a derate mode. In some cases, there may be no immediate symptoms, but the fault code serves as a warning of potential issues with the aftertreatment system.

How Technicians Diagnose This Fault

Diagnosing the SPN 625 FMI 17 fault code involves a systematic approach:

Start with a visual inspection of the ACM2 and its connectors for any signs of physical damage or corrosion.
Check the wiring harness for continuity and any signs of damage or corrosion that could disrupt communication.
Inspect and reseat all connectors to ensure they are properly connected and secure.
Perform CAN bus diagnostics using a scan tool to identify any communication errors or faults within the network.
Verify the integrity of the terminating resistors on the CAN network to ensure proper signal termination.

Technicians should use multimeters and diagnostic software to verify signal readings and ensure all electrical connections are intact before considering component replacement.

Common Misdiagnosis

One common misdiagnosis is assuming the ACM2 is faulty without thoroughly checking the wiring or connectors. Technicians may also mistakenly replace sensors unrelated to communication issues or overlook problems within the CAN bus network. It's crucial to follow a structured diagnostic process to avoid unnecessary part replacements.

Can You Keep Driving?

While the SPN 625 FMI 17 fault code is a pre-warning, continued driving without addressing the issue can lead to more severe problems, such as engine derate or increased emissions. It's advisable to have the fault diagnosed and repaired promptly to prevent further complications.

Mechanic's Diagnostic Tip

When dealing with communication faults like SPN 625 FMI 17, always start by checking the simplest components first, such as connectors and wiring. These are often the root cause of communication issues and can save time and resources if addressed early in the diagnostic process.

Frequently Asked Questions

Q1: Can a faulty ACM2 cause other fault codes?
A1: Yes, a faulty ACM2 can trigger additional fault codes related to the aftertreatment system due to disrupted communication.

Q2: How can I prevent SPN 625 FMI 17 from occurring?
A2: Regular maintenance, including inspecting and cleaning connectors and wiring, can help prevent communication issues.

Q3: Is it safe to clear the fault code without repairs?
A3: Clearing the fault code without addressing the underlying issue is not recommended, as it may lead to recurring problems and potential engine derate.