What is SPN 6588 FMI 31 & How to Solve It?

What is SPN 6588 FMI 31:

SPN 6588 refers to a general emissions control system fault, and FMI 31 indicates an abnormal but undefined condition.

This is often triggered when emissions-related components don’t match expected operating behavior, but no direct cause is identified.

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Why This Happens

• Multiple small faults triggering system confusion.
• Sensor values drifting or responding slowly.
• Outdated ECM calibration.
• DEF system underperforming without triggering specific code.
• Temporary data dropout in sensor communication.

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How to Fix It

1. Scan for other related aftertreatment fault codes.
2. Run a full emissions system check using OEM diagnostics.
3. Update ECM to latest software revision.
4. Replace any sensor showing unstable or slow data.
5. Clear the code and retest under normal operating conditions.

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How to Avoid It Before It Happens

1. Keep emissions software updated.
2. Replace sensors with known drift issues proactively.
3. Check data response rates during regen.
4. Don’t ignore minor DEF or regen faults, as they often lead to this.