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SPN 3031 FMI 3 – DEF Tank Temperature Sensor Circuit Failed High Fault Code (Symptoms, Causes & Fix)

The SPN 3031 FMI 3 fault code indicates a problem with the DEF tank temperature sensor circuit, specifically that the circuit is reading higher than expected. This can affect the DEF system's ability to function correctly, potentially leading to emissions issues.

What SPN 3031 FMI 3 Means

This code signifies that the DEF tank temperature sensor circuit is reporting a voltage higher than the acceptable range. This could be due to a sensor failure, wiring issues, or connector problems.

Common Causes of This Fault Code

  • DEF tank temperature sensor failure
  • Open or short in the DEF temperature sensor circuit
  • Corroded or damaged connectors
  • Wiring harness issues such as chafing or pinched wires

Symptoms Drivers May Notice

  • Check engine light illuminated
  • Possible reduction in engine performance
  • Increased DEF consumption
  • Potential for vehicle to enter limp mode

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 and connectors for signs of damage, corrosion, or loose connections.
  4. Perform a continuity test on the sensor circuit to identify any open or short circuits.
  5. Use a multimeter to measure the resistance of the DEF tank temperature sensor and compare it to the manufacturer's specifications.
  6. Check for any related fault codes that might indicate a broader issue with the DEF system.

Common Misdiagnosis

  • Replacing the DEF tank temperature sensor without checking the wiring and connectors
  • Assuming a DEF system issue without verifying the sensor circuit integrity
  • Overlooking intermittent wiring issues due to vibration or movement

Can You Keep Driving?

While it may be possible to continue driving with this fault code, it is not advisable. The DEF system is critical for emissions control, and ignoring the issue could lead to further engine performance problems or emissions compliance issues.

Mechanic's Diagnostic Tip

Always start with a thorough inspection of the wiring and connectors before replacing any components. Many issues with this fault code are due to simple electrical faults rather than sensor failures. Ensure all connections are clean and secure, and use a multimeter to verify circuit integrity.