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SPN 3226 FMI 20

Aftertreatment 1 Outlet NOx — Data drifted high.
The value has slowly crept higher than expected over time.

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

What FMI 20 usually points to: Sensor aging, contamination, or an offset problem. More on FMI 20.

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 20 mean?

Aftertreatment 1 Outlet NOx — data drifted high. The value has slowly crept higher than expected over time.

Is SPN 3226 FMI 20 serious?

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

What should I check first for SPN 3226 FMI 20?

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