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SPN 4364 FMI 1

Aftertreatment 1 SCR Conversion Efficiency — Value too low (most severe).
The reading is valid but far below the normal operating range, at the most serious 'too low' level.

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 1 usually points to: A real under-condition such as very low oil pressure or low fuel pressure, or a sensor that has drifted low. More on FMI 1.

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 1 mean?

Aftertreatment 1 SCR Conversion Efficiency — value too low (most severe). The reading is valid but far below the normal operating range, at the most serious 'too low' level.

Is SPN 4364 FMI 1 serious?

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

What should I check first for SPN 4364 FMI 1?

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