What is SPN 5580 FMI 18 & How to Solve It?

What is SPN 5580 FMI 18:

SPN 5580 refers to the Aftertreatment 1 Diesel Exhaust Fluid (DEF) Quality Sensor, and FMI 18 means the sensor is giving a valid reading that’s above the normal range.

This fault code means the DEF quality sensor is detecting DEF that may be too pure or chemically inconsistent. It usually points to improper DEF composition, which can affect emission system performance and trigger engine derate.

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Why This Happens

• DEF fluid that doesn’t meet ISO 22241 standards.
• Contaminated or diluted DEF in the tank.
• DEF sensor failure or incorrect calibration.
• Refilling DEF from an open or unsealed container.
• Software misinterpretation of fluid readings.

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How to Fix It

1. Drain and refill the DEF tank with certified, fresh DEF.
2. Check the DEF quality sensor for contamination or damage.
3. Verify all DEF connections are sealed properly.
4. Test DEF sample with a refractometer if available.
5. If high readings persist, replace the DEF quality sensor.
6. Clear the fault and confirm that normal operation resumes.

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How to Avoid It Before It Happens

1. Always use certified DEF stored in sealed, clean containers.
2. Train staff to inspect DEF seals and expiration dates before refilling.
3. Keep DEF tanks capped and free from water or dust intrusion.
4. Replace DEF sensors at recommended intervals.
5. Use a system like Clue to track fluid issues and alert your team before bad DEF affects system performance.