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Cummins SPN 4331 FMI 16: Meaning, Causes & Fix

Aftertreatment 1 Outlet NOx1 — NOx level at SCR outlet above threshold, indicating insufficient NOx conversion

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

Quick Answer

SPN 4331 FMI 16 = Outlet NOx Above Threshold
Severity: ⚠️ WARNING · System: SCR System / NOx Sensor · ✅ Can continue with caution

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Diagnostic Reference

Field Details
Code SPN 4331 FMI 16
Protocol J1939 SPN: 4331 FMI: 16
Component SCR System / NOx Sensor
Manufacturer Cummins
Engine Series ISX15
Severity ⚠️ WARNING
SAE Reference SAE J1939-73 Digital Annex — SPN 4331, FMI 16

Possible Causes

  1. DEF injection insufficient
  2. SCR catalyst degraded
  3. Outlet NOx sensor failure
  4. DEF quality below specification
  5. Exhaust gas temperature too low for SCR reaction

Diagnostic & Repair Procedure

  1. Step 1: Check DEF injection rate with INSITE diagnostic tool
  2. Step 2: Test outlet NOx sensor accuracy — compare to inlet sensor
  3. Step 3: Verify DEF quality (32.5% urea) and tank level
  4. Step 4: Monitor exhaust gas temperature during operation
  5. Step 5: If SCR catalyst is degraded, schedule catalyst replacement

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 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 4331 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.