SPN 625 FMI 8 – Motor Control Module, Powertrain Controller Area Network DM1 Message Not Received or has Stopped Arriving Fault Code (Symptoms, Causes & Fix)

Mar 12, 2026

The SPN 625 FMI 8 fault code indicates a communication issue within the diesel engine's control system. Specifically, it means that the Motor Control Module (MCM) is not receiving the DM1 message from the Powertrain Controller Area Network (CAN), or the message has stopped arriving. This diagnostic trouble code highlights a disruption in the expected data exchange between critical control modules.

What SPN 625 FMI 8 Means

When the SPN 625 FMI 8 fault code is active, it signifies a breakdown in communication between the MCM and the Powertrain Control Module (PCM) over the CAN network. The ECM interprets this as a failure to receive necessary operational data, which can affect engine performance and diagnostics. This fault is primarily related to electrical communication rather than mechanical issues.

Common Causes of This Fault Code

Faulty or damaged wiring harness between the MCM and PCM
Defective MCM or PCM
Corroded or loose connectors at the MCM or PCM
Intermittent power supply to the MCM or PCM
Software issues or outdated firmware in the MCM or PCM

Symptoms Drivers May Notice

When the SPN 625 FMI 8 fault code is present, drivers might experience symptoms such as engine warning lights on the dashboard, reduced engine performance, or erratic engine behavior. In some cases, the truck may enter a derate mode, limiting power to prevent further damage.

How Technicians Diagnose This Fault

Diagnosing the SPN 625 FMI 8 fault involves a systematic approach:

Start with a visual inspection of the wiring harness and connectors between the MCM and PCM for any signs of damage or corrosion.
Check the battery voltage and overall system voltage to ensure proper power supply.
Verify the integrity of the power and ground circuits at the MCM and PCM using a multimeter.
Conduct a CAN network integrity test with diagnostic software to confirm communication between modules.
Use a scan tool to check for additional fault codes that might indicate broader communication issues.
If necessary, update or reflash the MCM and PCM software to the latest version.

Common Misdiagnosis

Technicians often misdiagnose this fault by assuming the issue lies with engine sensors rather than the communication network. Another common mistake is replacing the MCM or PCM without first verifying the integrity of the wiring and connectors. Overlooking intermittent power supply issues due to corroded battery terminals or faulty fuses can also lead to misdiagnosis.

Can You Keep Driving?

While it may be possible to continue driving with the SPN 625 FMI 8 fault code active, it's not advisable. The communication issue can lead to engine derate or protection mode, which can significantly impact performance and potentially cause further damage. It's best to address the fault promptly to avoid these risks.

Mechanic's Diagnostic Tip

Always verify the integrity of the wiring and connectors before considering module replacement. Many communication issues stem from simple electrical faults that can be resolved without replacing expensive components. Pay close attention to any signs of corrosion or damage in the connectors, as these are common failure points.

Frequently Asked Questions

Q: What tools are essential for diagnosing SPN 625 FMI 8?
A: A multimeter and diagnostic software are crucial for checking voltage, continuity, and network integrity.

Q: Can software updates resolve SPN 625 FMI 8?
A: Yes, outdated firmware can cause communication issues, so updating the MCM and PCM software may resolve the fault.

Q: Is it common for this fault to cause engine derate?
A: Yes, communication failures can trigger engine derate to protect the engine from potential damage due to lack of data.