Code Details
| Display code | SPN 3361 FMI 1 |
|---|---|
| SPN | 3361 |
| FMI | 1 |
| OEM code | None listed |
| Manufacturer | Cummins |
| System | Aftertreatment �?DEF supply system |
| Component | DEF supply pump / DEF lines / filter |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The DEF supply pump draws fluid from the tank and maintains the pressure needed to deliver DEF to the dosing injector. When supply pressure falls below the normal threshold, DEF delivery may be inadequate, reducing SCR efficiency. Low pressure can result from a blocked DEF filter, a restricted supply line, frozen DEF (in cold weather), or a failing supply pump.
SPN 3361 is Aftertreatment DEF Dosing Unit Pressure. FMI 1 indicates a valid reading below the normal operating range. On Cummins CM2350 ISX15/X15, the DEF supply pressure is monitored at the pump module outlet. Insite can display the commanded and actual DEF pump pressure, helping distinguish a restriction from a pump output fault.
Common Symptoms
- SCR efficiency fault (SPN 5246 FMI 31) may accompany this code
- DEF system warning on the instrument cluster
- In cold weather, may follow a DEF freeze-thaw cycle if lines are still partially frozen
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Plugged DEF supply filter
- DEF supply line restricted or blocked
- DEF partially frozen in cold weather operation
- DEF supply pump motor worn or failing
- DEF pressure sensor fault
First Checks
- Check the DEF filter and replace if overdue or if the truck has been run with poor-quality or contaminated DEF
- In cold weather, allow the DEF system warm-up cycle to complete before evaluating the fault
- Connect Cummins Insite and monitor DEF supply pressure during a pump cycle
- Inspect DEF supply lines for kinks, blockages, or leaks
- If filter and lines are clear and pressure remains low, test the DEF pump output
Can I Keep Driving?
Driveable, but inadequate DEF supply will reduce SCR efficiency and may trigger the inducement sequence. Address promptly.
Related Codes
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
How often should the DEF supply filter be replaced on the Cummins ISX15 or X15?
Cummins specifies DEF filter replacement intervals based on mileage or hours in the maintenance schedule, typically in conjunction with DEF system inspections. The filter is located in the DEF pump module. Using quality ISO 22241 DEF reduces the rate at which the filter accumulates deposits. Always replace the DEF filter when servicing for DEF system pressure faults.
Can frozen DEF cause SPN 3361 FMI 1 on the ISX15 even if the DEF heater is working?
Yes. In extreme cold, the DEF system heater may not fully thaw all components before the engine reaches operating temperature and the ECM begins requesting DEF dosing. If the supply line or pump module is still partially frozen, supply pressure will be low until the system fully thaws. Allowing additional warm-up time in very cold conditions is the correct response for a first-occurrence pressure fault in cold weather.
Is the DEF pump on the ISX15 the same module as the quality sensor and level sensor?
On many DEF system designs used with the ISX15 and X15, the DEF pump, quality sensor, level sensor, and heater are integrated into a single module inside the DEF tank. This combined module is typically replaced as a unit when pump or sensor faults confirm internal failure. Insite can test each sensor and the pump output within the module to confirm which component has failed before recommending replacement.