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SPN 3361 FMI 4

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

What the SPN covers: The DEF doser (injector) that sprays diesel exhaust fluid into the exhaust ahead of the SCR catalyst. More on SPN 3361.

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

HIGH — limit use, diagnose now

A doser that is stuck, clogged or not firing means NOx is not being reduced, and inducement derates will follow. Repair soon.

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. White urea crystals blocking the doser tip or mixer
  3. Doser coil open or shorted
  4. DEF wicked into the doser connector or harness
  5. Doser stuck open and dribbling after shutdown

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. Pull the doser and inspect the tip and mixer pipe for crystal buildup
  4. Run the OEM dosing quantity (cup) test and compare with spec
  5. Measure coil resistance and inspect the connector for DEF intrusion
  6. Soak light deposits off in warm water; replace the doser if spray or volume stays wrong

FAQ

What does SPN 3361 FMI 4 mean?

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

Is SPN 3361 FMI 4 serious?

A doser that is stuck, clogged or not firing means NOx is not being reduced, and inducement derates will follow. Repair soon.

What should I check first for SPN 3361 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; Pull the doser and inspect the tip and mixer pipe for crystal buildup.