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SPN 4364 FMI 18

Aftertreatment 1 SCR Conversion Efficiency — Value too low (moderately severe).
The reading is below normal at a middle warning level — more serious than FMI 17, less than FMI 1.

What the SPN covers: How well the SCR system is reducing NOx compared with what it should be doing. More on SPN 4364.

What FMI 18 usually points to: A developing under-condition that often precedes a derate. More on FMI 18.

Severity

HIGH — limit use, diagnose now

Low SCR efficiency leads to inducement derates and a speed limit. Diagnose soon.

Likely causes

  1. Poor DEF quality
  2. DEF doser clogged or crystallized
  3. Faulty NOx sensor
  4. Exhaust leak
  5. Aged or contaminated SCR catalyst

Checks & fixes (easiest first)

  1. Test DEF concentration
  2. Inspect and clean the doser for crystals
  3. Run the OEM DEF dosing quantity test
  4. Verify NOx sensor readings
  5. Check for exhaust leaks

FAQ

What does SPN 4364 FMI 18 mean?

Aftertreatment 1 SCR Conversion Efficiency — value too low (moderately severe). The reading is below normal at a middle warning level — more serious than FMI 17, less than FMI 1.

Is SPN 4364 FMI 18 serious?

Low SCR efficiency leads to inducement derates and a speed limit. Diagnose soon.

What should I check first for SPN 4364 FMI 18?

Test DEF concentration; Inspect and clean the doser for crystals; Run the OEM DEF dosing quantity test.