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Cummins SPN 3031 FMI 16: Meaning, Causes & Fix

Aftertreatment 1 DEF Tank Level Low — DEF tank fluid level below minimum required for SCR system operation

Reviewed by ASE Certified Mechanics · Last updated July 08, 2026

Quick Answer

SPN 3031 FMI 16 = DEF Tank Level Low
Severity: 🟡 CHECK SOON · System: DEF Tank Level Sensor · ✅ Can continue with caution

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Diagnostic Reference

Field Details
Code SPN 3031 FMI 16
Protocol J1939 SPN: 3031 FMI: 16
Component DEF Tank Level Sensor
Manufacturer Cummins
Engine Series ISX15
Severity 🟡 CHECK SOON
SAE Reference SAE J1939-73 Digital Annex — SPN 3031, FMI 16

Possible Causes

  1. DEF tank empty or near empty
  2. DEF level sensor failure
  3. DEF tank leak
  4. DEF line leak causing loss
  5. Wiring issue to level sensor

Diagnostic & Repair Procedure

  1. Step 1: Refill DEF tank with certified DEF immediately
  2. Step 2: Check for DEF tank or line leaks under vehicle
  3. Step 3: Test DEF level sensor operation with INSITE
  4. Step 4: If tank is full but code persists — test sensor and wiring
  5. Step 5: Do not operate vehicle without adequate DEF level

Frequently Asked Questions

Is this a false alarm or a real problem?

CHECK SOON codes are rarely false alarms. The ECM has detected a parameter outside normal operating range. Sensor malfunctions can sometimes trigger false readings, but the majority of CHECK SOON codes indicate a developing issue that will worsen if left unaddressed. Use a diagnostic scan tool to verify sensor readings before replacing expensive parts.

Can I diagnose this without a scan tool?

Basic visual inspection can be performed without a scan tool — check wiring harnesses for chafing, connectors for corrosion, and related components for physical damage. However, to verify the fault, view live sensor data, and confirm the repair, a heavy-duty diagnostic scan tool (or a compatible OBD-II adapter with J1939/J1708 support) is strongly recommended for accurate diagnosis.

How soon should I schedule service?

Schedule service within the next 1–3 operating days. While the vehicle can continue operating normally, delaying repair increases the risk of the fault escalating to CHECK NEXT STOP severity. Additionally, some CHECK SOON conditions are precursors to emissions system faults that will trigger a mandatory derate if not addressed in a timely manner.

Estimated Repair Cost

Typical repair: $100–800 (Parts: $50–500 + Labor: 1–3 hours) · Costs vary by make/model and location

Diesel Repair Cost & Downtime Estimator

Enter estimated labor hours to calculate repair cost and potential fleet downtime losses. Based on national average diesel shop rate of $150/hr and daily revenue loss of $800/day for a parked truck.

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SPN 639 FMI 9 🔴 STOP ENGINE
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SPN 1322 FMI 31 🟡 CHECK AT NEXT STOP
Misfire Detected on Cylinder 6
MID 128 SID 1 FMI 5 🟡 CHECK AT NEXT STOP
Injector Cylinder 1 Circuit

Associated Symptoms

Engine Derate Loss Of Power

References & Further Reading

Data Provenance: This fault code definition is derived from SAE J1939 standards. SPN 3031 FMI 16 is defined in SAE J1939-73 Digital Annex. Diagnostic procedures sourced from Cummins OEM technical service documentation. Not for safety-critical decisions. Consult a certified diesel mechanic before performing any repair. See our full disclaimer.