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SPN 3226 FMI 2

Aftertreatment 1 Outlet NOx — Data erratic, intermittent, or incorrect.
The signal jumps around, drops in and out, or disagrees with other information the controller has.

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

What FMI 2 usually points to: Loose or corroded connectors, chafed wiring, a failing sensor, or two redundant sensors that do not agree. More on FMI 2.

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 2 mean?

Aftertreatment 1 Outlet NOx — data erratic, intermittent, or incorrect. The signal jumps around, drops in and out, or disagrees with other information the controller has.

Is SPN 3226 FMI 2 serious?

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

What should I check first for SPN 3226 FMI 2?

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