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Cummins SPN 3226 FMI 4: Meaning, Causes & Fix

Aftertreatment Outlet NOx Sensor Circuit Voltage Low — outlet NOx sensor signal below normal, indicating sensor or wiring failure

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

Quick Answer

SPN 3226 FMI 4 = Outlet NOx Sensor Voltage Low
Severity: ⚠️ WARNING · System: Aftertreatment Outlet NOx Sensor · ✅ Can continue with caution

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Diagnostic Reference

Field Details
Code SPN 3226 FMI 4
Protocol J1939 SPN: 3226 FMI: 4
Component Aftertreatment Outlet NOx Sensor
Manufacturer Cummins
Engine Series ISX15
Severity ⚠️ WARNING
SAE Reference SAE J1939-73 Digital Annex — SPN 3226, FMI 4

Possible Causes

  1. NOx sensor failure
  2. Sensor wiring harness damage
  3. Corroded sensor connector
  4. Sensor heater circuit malfunction
  5. ECM power supply issue

Diagnostic & Repair Procedure

  1. Step 1: Test NOx sensor voltage at the connector
  2. Step 2: Inspect wiring harness for damage or chafing
  3. Step 3: Check sensor connector for corrosion or loose pins
  4. Step 4: Replace NOx sensor if voltage is out of specification
  5. Step 5: Verify ECM power supply to sensor heater circuit

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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Associated Symptoms

Engine Derate Loss Of Power

References & Further Reading

Data Provenance: This fault code definition is derived from SAE J1939 standards. SPN 3226 FMI 4 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.