What is SPN 5392 FMI 31 & How to Solve It?

What is SPN 5392 FMI 31:

SPN 5392 refers to a General Emissions System fault, and FMI 31 means the issue is detected but not specifically defined — usually pointing to an abnormal condition without a direct source.

SPN 5392 FMI 31 often shows up when multiple emission-related sensors or components are showing inconsistent data, causing the system to trigger a general fault.

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Why This Happens

Common causes include:

• Multiple sensor signals out of range (e.g., NOx, DPF temp, DEF dosing).
• DEF dosing errors not tied to one part.
• Software misalignment or timing issues.
• Poor-quality DEF affecting readings across components.
• Communication loss between the ECM and emission control modules.

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How to Fix It

1. Scan for additional fault codes that can help isolate the issue.
2. Inspect all major aftertreatment sensors: NOx, temp, pressure, DEF.
3. Check wiring harnesses for loose connections or damage.
4. Perform a regen to verify sensor operation.
5. If no single issue is found, update or reflash control module software.
6. Clear the fault and retest the system after restart.

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How to Avoid It Before It Happens

1. Stick to the maintenance schedule for your full emissions system.
2. Train operators to report early warning signs like regen failures.
3. Avoid mixing DEF brands or using poor-quality fluid.
4. Use diagnostic tools weekly to catch developing issues.