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SPN 4334 FMI 18

Aftertreatment 1 DEF Doser Absolute Pressure — 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: DEF pressure in the line between the DEF pump and the doser, used to confirm the pump is priming and holding pressure. More on SPN 4334.

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

If the pump cannot build pressure, DEF is not dosing and inducement derates will follow.

Likely causes

  1. Air leak or worn O-rings on the suction line or tank header
  2. Clogged DEF pump filter or tank strainer
  3. Frozen or crystallized line
  4. Failed DEF pump
  5. Restricted or kinked return line

Checks & fixes (easiest first)

  1. Replace the DEF pump filter
  2. Run the pump prime test and watch whether pressure builds and holds
  3. Temporarily run a clear suction line to look for air bubbles
  4. Inspect lines for kinks, crystals or freezing
  5. Check pressure sensor reading key-on, engine-off

FAQ

What does SPN 4334 FMI 18 mean?

Aftertreatment 1 DEF Doser Absolute Pressure — 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 4334 FMI 18 serious?

If the pump cannot build pressure, DEF is not dosing and inducement derates will follow.

What should I check first for SPN 4334 FMI 18?

Replace the DEF pump filter; Run the pump prime test and watch whether pressure builds and holds; Temporarily run a clear suction line to look for air bubbles.