SPN 625 FMI 4 – Erratic data or the Controller Area Network Propriety Data Link has Failed Between the Motor Control Module and the Common Powertrain Controller Fault Code (Symptoms, Causes & Fix)

Mar 12, 2026

The SPN 625 FMI 4 fault code indicates an issue with erratic data or a failure in the Controller Area Network (CAN) proprietary data link between the Motor Control Module (MCM) and the Common Powertrain Controller (CPC). This diesel engine fault code suggests that the ECM is receiving inconsistent or no communication from these critical components, which can affect engine performance.

What SPN 625 FMI 4 Means

SPN 625 FMI 4 represents a communication error between the MCM and CPC. The ECM interprets this as either erratic data or a complete failure in the CAN data link. This fault can disrupt the flow of information necessary for optimal engine control, potentially leading to performance issues.

Common Causes of This Fault Code

Faulty wiring harness or connectors between the MCM and CPC
Defective Motor Control Module (MCM)
Defective Common Powertrain Controller (CPC)
Intermittent CAN network issues
Corrosion at connectors

Symptoms Drivers May Notice

Drivers may experience symptoms such as engine hesitation, reduced power, or erratic engine behavior. In some cases, the engine may enter a derate mode, limiting power to protect the engine from potential damage.

How Technicians Diagnose This Fault

Technicians begin by performing a visual inspection of the wiring harness and connectors for any signs of damage, corrosion, or loose connections. Using a multimeter, they check for continuity and resistance in the wiring between the MCM and CPC. The CAN network is inspected for proper termination and resistance using diagnostic tools. If necessary, swapping the MCM or CPC with a known good unit can help determine if the fault follows the module. Technicians also check for any related fault codes that may provide additional context.

Common Misdiagnosis

A common mistake is replacing the MCM or CPC without verifying the integrity of the wiring harness and connectors. Technicians may also overlook intermittent wiring issues that only occur under certain conditions or assume a software issue without checking hardware connections.

Can You Keep Driving?

While it may be possible to continue driving with this fault, it is not advisable. The fault can lead to engine derate or protection mode, which can significantly impact performance and potentially cause further damage if not addressed promptly.

Mechanic's Diagnostic Tip

Always verify the integrity of the wiring harness and connectors before replacing any modules. Intermittent issues can often be traced back to poor connections or damaged wiring, which are more cost-effective to repair than replacing expensive components unnecessarily.

Frequently Asked Questions

Q: Can a software update fix SPN 625 FMI 4?
A: While a software update may occasionally resolve communication issues, it is crucial to first verify the physical connections and wiring integrity.

Q: How can I prevent SPN 625 FMI 4 from occurring?
A: Regular maintenance checks on wiring harnesses and connectors for signs of wear or corrosion can help prevent this fault.

Q: Is SPN 625 FMI 4 a common fault?
A: It is not uncommon, especially in older vehicles where wiring and connectors may have degraded over time.