SPN 3250 FMI 31 – Abnormal DOC Temperature Rise 1 Fault Code (Symptoms, Causes & Fix)

Sep 13, 2026

The SPN 3250 FMI 31 fault code indicates an abnormal temperature rise in the Diesel Oxidation Catalyst (DOC) of a diesel engine. This diagnostic trouble code is triggered when the Engine Control Module (ECM) detects a temperature reading from the DOC that exceeds expected parameters, suggesting potential issues with the aftertreatment system.

What SPN 3250 FMI 31 Means

The SPN 3250 FMI 31 fault code specifically pertains to the Diesel Oxidation Catalyst (DOC) temperature sensor. The ECM monitors the temperature to ensure the DOC is operating within its designed range. An abnormal rise in temperature can indicate problems such as excessive soot accumulation, faulty sensors, or issues with the fuel dosing system during regeneration.

Common Causes of This Fault Code

Faulty DOC temperature sensor
Wiring harness issues (shorts or opens)
Excessive soot accumulation in DOC
Exhaust leaks upstream of DOC
Incorrect fuel dosing during regeneration

Symptoms Drivers May Notice

When the SPN 3250 FMI 31 fault code is active, drivers may notice a reduction in engine performance, increased exhaust smoke, or a warning light on the dashboard. In some cases, the engine may enter a derate mode to protect the aftertreatment system from damage.

How Technicians Diagnose This Fault

Technicians begin by performing a visual inspection of the wiring harness and connectors for any signs of damage or corrosion. Next, they verify the power supply and ground circuits to the DOC temperature sensor using a multimeter. Checking for proper voltage reference is crucial to rule out electrical faults. The sensor's signal readings are then verified using diagnostic software to ensure accurate data is being sent to the ECM. If necessary, a soot load test is conducted to assess the level of accumulation in the DOC.

Common Misdiagnosis

A common mistake is assuming the DOC itself is faulty without verifying the operation of the temperature sensor. Technicians may also replace the sensor without checking the integrity of the wiring harness, leading to repeated issues. Overlooking exhaust leaks that can affect temperature readings is another frequent oversight.

Can You Keep Driving?

While it may be possible to continue driving with the SPN 3250 FMI 31 fault code active, it is not advisable. The fault can lead to engine derate or protection mode, reducing performance and potentially causing further damage to the aftertreatment system. Prompt diagnosis and repair are recommended to avoid these issues.

Mechanic's Diagnostic Tip

When diagnosing SPN 3250 FMI 31, always verify the DOC temperature sensor's operation before considering replacement. Use a multimeter to check for proper voltage and continuity in the wiring harness to avoid unnecessary part replacements.

Frequently Asked Questions

What causes SPN 3250 FMI 31? This fault code is commonly caused by a faulty DOC temperature sensor, wiring issues, excessive soot in the DOC, or incorrect fuel dosing during regeneration.
Can a faulty DOC temperature sensor trigger other codes? Yes, a malfunctioning sensor can lead to additional fault codes related to the aftertreatment system.
How can I prevent SPN 3250 FMI 31 from occurring? Regular maintenance of the exhaust aftertreatment system, including cleaning the DOC and ensuring proper sensor operation, can help prevent this fault code.