Truck Fault Codes

Cummins ISB 6.7 SPN 3361 FMI 1 — DEF Supply Pressure Below Normal

SPN 3361 FMI 1 on a Cummins ISB 6.7 means the DEF supply pump is not maintaining adequate pressure for DEF delivery. Check the DEF filter for restriction, inspect supply lines for blockage or freezing, and confirm DEF level and quality. Cummins INSITE can monitor live DEF supply pressure to identify whether the issue is a restriction or a failing pump.

Code Details

Structured details for SPN 3361 FMI 1
Display codeSPN 3361 FMI 1
SPN3361
FMI1
OEM codeNone listed
ManufacturerCummins
SystemAftertreatment — DEF supply system
ComponentDEF supply pump / DEF lines / filter
Source addressUnknown or not applicable
Severitymedium
Review statussource backed
Source confidencemedium
Last reviewed2026-06-12

Plain-English Meaning

The Cummins ISB 6.7 uses a DEF supply pump to draw fluid from the tank and deliver it under pressure to the dosing injector. When pressure falls below the normal range, DEF cannot be delivered accurately to the SCR catalyst, reducing NOx conversion efficiency. The ISB 6.7 is used in medium-duty trucks, school buses, and vocational applications where smaller DEF tanks and shorter route profiles can increase the likelihood of DEF system faults. Cummins INSITE is the appropriate diagnostic tool for identifying the cause of the supply pressure fault.

SPN 3361 is related to Aftertreatment DEF Dosing Unit Pressure in J1939. FMI 1 indicates a below-normal reading. On Cummins CM2350 ISB 6.7 engines, the DEF supply module includes the pump, level sensor, quality sensor, and heater in an integrated assembly. The ACM monitors supply pressure continuously; when it falls below the calibrated threshold, SPN 3361 FMI 1 is logged. INSITE provides commanded and actual DEF pump pressure data and supports pump activation tests.

Common Symptoms

  • SCR efficiency warning or DEF system fault on the instrument panel
  • DEF dosing activity below commanded levels in INSITE
  • Possible cold-weather occurrence before DEF system warm-up is complete
  • Inducement sequence may begin if DEF delivery is insufficient for extended operation
  • Companion code SPN 3480 FMI 5 may appear if pressure fault leads to dosing injector activation errors

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

  • Plugged DEF supply filter
  • DEF supply line blocked, kinked, or frozen
  • DEF supply pump degraded or failing
  • DEF pressure sensor fault
  • Contaminated DEF restricting the filter and supply system

First Checks

  • Replace the DEF filter as an early step in diagnosis
  • In cold weather, ensure the DEF heater has completed its warm-up cycle before pursuing further diagnosis
  • Connect Cummins INSITE and run a DEF pump activation test to monitor supply pressure
  • Inspect DEF supply lines for kinks, damage, or freezing
  • Verify DEF quality meets ISO 22241 specification
  • If lines and filter are clear and pressure remains low, test the DEF supply module for pump output

Can I Keep Driving?

The ISB 6.7 can initially continue operating with SPN 3361 FMI 1 active, but the SCR system will not dose effectively and the inducement sequence may begin. Address the supply pressure fault promptly to prevent escalation.

SPN 3480 FMI 5Aftertreatment — DEF dosing system · DEF dosing injector / wiring circuit

Related Lookup Pages

Sources

  • Cummins INSITE Service Tool — Public Reference Documentation Cummins Inc. · oem · accessed 2026-06-11 · confidence medium

    Source: Cummins Inc., Cummins INSITE Service Tool — Public Reference Documentation. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source
  • QuickServe Online Cummins Inc. · oem · accessed 2026-05-05 · confidence medium

    Source: Cummins Inc., QuickServe Online. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source
  • SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium

    Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source

FAQ

Does the ISB 6.7 share DEF system components with the ISX15?

The ISB 6.7 and ISX15 both use Cummins CM2350 ECM architecture and a similar DEF system structure, but the modules and component sizes differ due to the difference in engine displacement and application. The diagnostic approach using INSITE is the same across both engines. Some DEF module part numbers are shared across different Cummins engine sizes, but a Cummins parts lookup by serial number confirms the exact specification.

Can the Cummins ISB 6.7 DEF pump fail prematurely on school bus or stop-and-go routes?

High-cycle duty in stop-and-go operation can affect DEF pump longevity. The DEF pump runs more frequently in duty cycles with short trips because the aftertreatment system must regen and dose more often relative to distance traveled. Using quality DEF and maintaining the filter at the recommended interval are the best practices for extending pump life on ISB 6.7 engines in high-demand applications.

Is INSITE required to reset the inducement after fixing SPN 3361 FMI 1 on the ISB 6.7?

If the fault clears on its own after the supply pressure is restored and a drive cycle is completed, no manual reset may be needed. If the inducement has advanced to a stage requiring a manual reset, Cummins INSITE is used to perform the reset procedure. The inducement counter stage and accumulated distance are visible in INSITE to help determine whether a manual reset is needed.