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SPN 3226 FMI 4

Aftertreatment 1 Outlet NOx — Voltage below normal or shorted low.
The sensor circuit voltage is lower than it should ever be, usually near zero.

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

What FMI 4 usually points to: Signal wire shorted to ground, a lost 5 V reference, or an open signal wire on a pulled-down circuit. More on FMI 4.

Severity

MEDIUM — fix soon

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

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Failed NOx sensor
  3. Real high NOx from poor DEF dosing
  4. Exhaust leak
  5. Sensor wiring or module fault

Checks & fixes (easiest first)

  1. Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins
  2. Check the reference voltage, ground and signal at the component per OEM pin-out
  3. Check DEF quality and dosing
  4. Check for exhaust leaks
  5. Check sensor wiring and power
  6. Run the OEM SCR performance test

FAQ

What does SPN 3226 FMI 4 mean?

Aftertreatment 1 Outlet NOx — voltage below normal or shorted low. The sensor circuit voltage is lower than it should ever be, usually near zero.

Is SPN 3226 FMI 4 serious?

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

What should I check first for SPN 3226 FMI 4?

Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins; Check the reference voltage, ground and signal at the component per OEM pin-out; Check DEF quality and dosing.