SPN 3226 FMI [2-20] Explained, Troubleshooting

SPN 3226 is a diagnostic trouble code for Aftertreatment 1 Outlet NOx.

FMI 2 – Data Erratic, Intermittent Or Incorrect

FMI 4 – Voltage Below Normal, Or Shorted To Low Source
FMI 9 – Abnormal Update Rate
FMI 10 – Abnormal Rate Of Change
FMI 13 – Out of Calibration
FMI 20 – Data Drifted High

NOx is nitrous oxide.


Troubleshoot:NOx sensor and circiut to ECM, wiring harness

  • Identify the issue: Note the specific fault code (SPN 3226) and any accompanying Failure Mode Identifier (FMI) codes that provide more detailed information about the issue.
    • FMI 3: Voltage Above Normal
  • Consult the manufacturer’s documentation: Refer to the engine manufacturer’s troubleshooting guide or service manual for information on the SCR system and any specific recommendations for addressing SPN 3226.
  • Perform a visual inspection: Check the SCR Outlet NOx sensor, its wiring, and connectors for visible damage, corrosion, or loose connections.
  • Inspect the NOx sensor: Use a diagnostic tool to verify that the SCR Outlet NOx sensor is providing accurate readings. Follow the manufacturer’s testing procedures, which may involve comparing the sensor output with known good values or other diagnostic steps.
  • Check the wiring and connectors: Examine the wiring and connectors associated with the SCR Outlet NOx sensor for signs of damage, corrosion, or loose connections. Repair or replace any damaged components as necessary.
  • Check for additional fault codes: Use a diagnostic tool to scan the engine control module (ECM) for any additional fault codes that may be related to the SCR system, such as DEF injector or SCR catalyst issues.
  • Verify repair: After identifying and addressing the cause of the problem, test the system to ensure that the SCR system is working correctly and that the SPN 3226 fault code has been resolved. Clear the fault codes with a diagnostic tool and run the engine to see if the code reappears.