SPN 4334 FMI 18 — DEF pump pressure below target
Applies to: Detroit · Navistar · Paccar · Sae · Volvo
DEF pump pressure below target.
Your SPN/FMI rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Caution. Limp to the next safe stop.
Most likely causes
- Failed DEF pump
- Clogged DEF filter
- Air leak in DEF supply
- Frozen DEF lines (winter)
- Low DEF level uncovering pickup
- Pump failed
- DEF filter clogged
First moves (roadside)
- Check DEF filter condition
- Verify pump runs at key-on
- Inspect lines for kinks/leaks/frost
- Check DEF level
- Check DEF filter
- Verify pump runs on key-on
- Inspect lines for kinks/leaks
Common misdiagnoses
- Replacing pump without checking filter first
FAQ
DEF pump pressure below target.
Caution — Limp to the next safe stop.
The most common cause is: Failed DEF pump. Other possibilities include Clogged DEF filter; Air leak in DEF supply; Frozen DEF lines (winter); Low DEF level uncovering pickup; Pump failed; DEF filter clogged.
Limp to the next safe stop. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.