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SPN 3031 FMI 4 – DEF Tank Temperature Sensor Circuit Failed Low Fault Code (Symptoms, Causes & Fix)

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The SPN 3031 FMI 4 fault code indicates a low signal from the DEF tank temperature sensor circuit. This typically points to an electrical issue such as a sensor failure or wiring problem within the aftertreatment system.

What SPN 3031 FMI 4 Means

This code is triggered when the Engine Control Module (ECM) detects that the DEF tank temperature sensor circuit is reading lower than expected. This could be due to an open circuit, short to ground, or a faulty sensor.

Common Causes of This Fault Code

  • DEF tank temperature sensor failure
  • Open or short circuit in the sensor wiring
  • Corroded or damaged connectors
  • Faulty ECM input

Symptoms Drivers May Notice

  • Check engine light illuminated
  • Possible derate in engine performance
  • Inaccurate DEF tank temperature readings

How Technicians Diagnose This Fault

  1. Check battery voltage and overall system voltage to ensure proper power supply.
  2. Verify the sensor's power supply and ground circuits for integrity.
  3. Inspect the wiring harness for continuity and any signs of damage, such as chafing or pinching.
  4. Test the connectors for corrosion or poor pin fitment.
  5. Use a multimeter to measure the voltage and resistance at the sensor and compare with manufacturer specifications.
  6. Perform an ECM pin-out test to ensure proper signal reception.

Common Misdiagnosis

  • Replacing the DEF tank temperature sensor without verifying wiring integrity.
  • Assuming a DEF quality issue without checking electrical components.
  • Overlooking connector corrosion as a potential cause.

Can You Keep Driving?

While the vehicle may still be operable, continued driving with this fault can lead to further issues, including potential derate or aftertreatment system inefficiencies. It's advisable to address the fault promptly to avoid additional complications.

Mechanic's Diagnostic Tip

Always verify the integrity of the wiring and connectors before replacing the sensor. Many issues stem from poor electrical connections rather than sensor failure itself. Use a scan tool to confirm the fault and ensure no other related codes are present.