What is SPN 593 FMI 31 & How to Solve It?

What is SPN 593 FMI 31:

SPN 593 relates to the Fuel Injection Control System, and FMI 31 indicates an abnormal but undefined condition.

This usually means that something in the fuel injection system — timing, pressure, or delivery — isn’t operating within expected parameters, but the system can’t pinpoint which part is causing the issue.

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Why This Happens

• Inconsistent injector performance.
• Poor fuel quality or air in the fuel lines.
• Wiring problem in injector control circuits.
• Early-stage high-pressure pump or ECM fault.
• Corrupted signal timing to injectors.

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How to Fix It

1. Run cylinder balance and injector performance tests.
2. Check fuel pressure and verify there’s no air in the lines.
3. Inspect injector harness for damage or loose pins.
4. Replace any injector with erratic spray pattern or response.
5. Clear the code and test system behavior under load.

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How to Avoid It Before It Happens

1. Use clean diesel and maintain fuel filtration systems.
2. Log injector data and watch for early inconsistencies.
3. Replace worn or slow injectors proactively.
4. Keep the fuel system air-free by monitoring the priming process.