You are currently viewing SPN 3226 FMI [2-20] Explained, Troubleshooting

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.



I was on the road, halfway through my route, when the dashboard lit up with an ominous check engine light. Accompanying it was an error code I hadn’t seen before: SPN 3226. That little orange light felt like a dark cloud looming over my day.

I pulled over at the next rest stop, a small clearing just off the highway surrounded by tall pine trees. The air was crisp, and the sound of passing traffic was a stark contrast to the silence inside my cab. I reached for the vehicle’s manual that was stuffed in the glove compartment, its pages worn from use. Flipping through the technical jargon, I located the section on error codes.

SPN 3226, as it turned out, related to the aftertreatment NOx sensor, which monitors the level of nitrogen oxide being emitted by the engine. This sensor played a crucial role in maintaining the truck’s compliance with environmental standards, and a fault here meant trouble not just for the vehicle’s performance but potentially for the environment too.

I stepped out of the cab and into the chill, my breath visible in the morning air. The truck, a robust beast of a machine with a 12.8-liter engine capable of 455 horsepower, now seemed vulnerable, its complexity laid bare by a single error code. I knew the aftertreatment system was located towards the back of the engine, a maze of components designed to reduce emissions.

Armed with a basic toolkit, I decided to take a look for myself. The NOx sensor was a small part, no more than a few inches long, but its impact was significant. I checked the connections, making sure they were secure and free of corrosion. Everything seemed to be in order, but the error persisted. I realized this was beyond a quick roadside fix.