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SPN 4334 FMI 4

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

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 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

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

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Air leak or worn O-rings on the suction line or tank header
  3. Clogged DEF pump filter or tank strainer
  4. Frozen or crystallized line
  5. Failed DEF pump
  6. Restricted or kinked return line

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

FAQ

What does SPN 4334 FMI 4 mean?

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

Is SPN 4334 FMI 4 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 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; Replace the DEF pump filter.