What is SPN 3242 FMI 3 & How to Solve It?

What is SPN 3242 FMI 3:

SPN 3242 refers to the Aftertreatment 1 Outlet NOx Sensor, and FMI 3 means the sensor is reading a voltage or value above the normal range.

This code usually indicates high NOx levels at the DPF outlet, which could mean the SCR system is not functioning properly or the sensor itself is faulty.

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Why This Happens

• DEF not dosing correctly.
• SCR catalyst degraded or bypassed.
• Outlet NOx sensor failing or reading incorrectly.
• ECM misinterpreting sensor data due to drift or delay.
• Excess engine load increasing NOx emissions.

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How to Fix It

1. Inspect and test the NOx outlet sensor with a scan tool.
2. Replace the sensor if values are unusually high with proper DEF dosing.
3. Test DEF quality and dosing rate.
4. Check for SCR damage or blockage.
5. Clear the code and verify outlet NOx levels during regen.

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How to Avoid It Before It Happens

1. Monitor NOx sensor behavior over time during service.
2. Replace sensors before they degrade beyond spec.
3. Maintain proper DEF dosing through regular cleaning and inspection.
4. Use diagnostic regen to verify SCR effectiveness.

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