Home › Codes › SPN 3226 › FMI 12

SPN 3226 FMI 12

Aftertreatment 1 Outlet NOx — Bad intelligent device or component.
A smart sensor, actuator, or module has reported an internal failure or is not responding correctly.

What the SPN covers: NOx sensor reading in the exhaust after the SCR catalyst (tailpipe NOx). More on SPN 3226.

What FMI 12 usually points to: Internal failure of the device itself; sometimes a power/ground problem makes a good device look bad. More on FMI 12.

Severity

MEDIUM — fix soon

A bad outlet sensor often triggers SCR efficiency faults and derates.

Likely causes

  1. Failed NOx sensor
  2. Real high NOx from poor DEF dosing
  3. Exhaust leak
  4. Sensor wiring or module fault

Checks & fixes (easiest first)

  1. Check DEF quality and dosing
  2. Check for exhaust leaks
  3. Check sensor wiring and power
  4. Run the OEM SCR performance test

FAQ

What does SPN 3226 FMI 12 mean?

Aftertreatment 1 Outlet NOx — bad intelligent device or component. A smart sensor, actuator, or module has reported an internal failure or is not responding correctly.

Is SPN 3226 FMI 12 serious?

A bad outlet sensor often triggers SCR efficiency faults and derates.

What should I check first for SPN 3226 FMI 12?

Check DEF quality and dosing; Check for exhaust leaks; Check sensor wiring and power.