Fault Code 3255 (SPN 3246 FMI 0 or 16) indicates that your DPF outlet temperature sensor is reporting dangerously high readings. If ignored, this can lead to aftertreatment damage or forced derate.

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What SPN 3246 FMI 0/16 Means

• SPN 3246 = DPF Outlet Temperature

• FMI 0 = Temperature above normal range

• FMI 16 = Data erratic, intermittent, or incorrect

Together, these mean the DPF outlet temp sensor has either exceeded its safe range or is sending unreliable data.

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Common Symptoms

• Check Engine Light or MIL illuminated

• Errors during parked regen attempts

• Erratic exhaust temperature readings

• Possible full engine derate if unresolved

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Likely Causes

• Faulty or degraded DPF outlet temp sensor

• Damaged wiring or connector corrosion

• Intermittent electrical shorts in the harness 

• Unusually high exhaust temperatures caused by airflow issues or regen failures

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Diagnosis and Repair Steps

1. Connect your diesel laptop to read SPN 3246 and live temperature data.

2. Monitor the DPF outlet temp during idle and regen—call out erratic behavior or spikes

3. Inspect wiring and connectors for damage, shorts, or corrosion

4. Replace the DPF outlet temp sensor if readings stay out of range or are unstable

5. After repair, clear the code and perform a forced regen to validate the fix

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Tools Required

• Diesel laptop

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Pro Tip

A sensor reading that jumps abruptly during regen likely points to a shorted or chafed harness, not necessarily a bad sensor

 Fix wiring before replacing the sensor.

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Final Summary

Fault Code 3255 (SPN 3246 FMI 0 or 16) is a critical signal that your DPF outlet temperature sensor is malfunctioning—either due to overheating or faulty wiring. Promptly diagnosing and addressing this issue with your diesel laptop helps avoid costly aftertreatment damage or full engine derates.