What SPN 4334 FMI 4 means
SPN 4334 FMI 4: Aftertreatment DEF pressure — voltage below normal / pressure too low.
Applies to: Cummins SCR engines 2010–2026.
Severity: High — dosing stops, SCR inducement follows.
Symptoms you'll see
- DEF lamp
- Dosing stops (visible in live data as zero dosing rate)
- SCR efficiency codes follow
Likely causes (ranked by probability)
- Failed DEF pump module (~40%)
- Clogged DEF filter (~30%) — maintenance item, often overdue
- Frozen DEF lines (winter — DEF freezes at 12°F) (~15%)
- Leaking or kinked DEF line (~10%)
Diagnostic steps
- Check ambient temperature first in winter — frozen DEF is a thaw, not a repair.
- Inspect/replace the DEF filter if service history is unknown.
- Command the DEF pump in INSITE and watch pressure response.
- Replace pump module if pressure won't build with a clean filter.
Tools that read, clear and verify this code
INSITE's dosing-system test sequence isolates pump vs filter vs line in one guided procedure. For the full per-platform picture see the Cummins ISX fault codes page and our SCR/DEF service guide.
Frequently Asked Questions
Can I drive with SPN 4334 FMI 4 active?
Severity on this code: High — dosing stops, SCR inducement follows. If a derate countdown is armed, use the remaining runtime to reach a shop — not to finish the week. Verify the root cause with the diagnostic steps above rather than clearing and hoping.
Will clearing SPN 4334 FMI 4 without repairs work?
The code will return — usually within one drive cycle — and on aftertreatment-related faults the inducement countdown often shortens each time. Repair first, verify with the guided test, then clear.
Which scanner reads SPN 4334 FMI 4?
INSITE's dosing-system test sequence isolates pump vs filter vs line in one guided procedure.