Code Details
| Display code | SPN 3361 FMI 1 |
|---|---|
| SPN | 3361 |
| FMI | 1 |
| OEM code | None listed |
| Manufacturer | PACCAR |
| 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 | 2026-06-12 |
Plain-English Meaning
The PACCAR MX-13 DEF system requires the supply pump to maintain adequate pressure to deliver DEF from the tank to the dosing injector. When supply pressure drops below the threshold, dosing becomes unreliable and the SCR catalyst does not receive enough reductant to convert NOx. The most common causes on MX-13 trucks are DEF filter restriction, a partially frozen supply line in cold weather, or a failing pump motor. Identifying and correcting the root cause is necessary to prevent the inducement sequence from progressing.
SPN 3361 is related to Aftertreatment DEF Dosing Unit Pressure in J1939. FMI 1 indicates a below-normal reading. On PACCAR MX-13 engines, the DEF supply module integrates the pump, filter, and associated sensors. The EMS monitors supply pressure and logs SPN 3361 FMI 1 when the measured value falls below the calibration threshold. PACCAR ESA provides live DEF pressure data and the ability to activate the supply pump for diagnostic testing.
Common Symptoms
- SCR efficiency fault or NOx conversion warning may appear alongside this code
- DEF dosing quantity below commanded value in PACCAR ESA data
- DEF or aftertreatment warning on the Kenworth or Peterbilt instrument cluster
- In cold weather, increased likelihood of a first-occurrence pressure fault before DEF system warm-up completes
- Inducement sequence progression if the pressure fault is not resolved
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Clogged DEF supply filter
- DEF supply line kinked, blocked, or partially frozen
- DEF supply pump motor degraded or failed
- DEF pressure sensor reading below actual pressure
- DEF pump check valve stuck or failed
First Checks
- Replace the DEF filter if overdue or if contaminated DEF has been used recently
- In cold weather, allow the DEF heater system time to fully thaw the supply path before evaluating the pressure fault
- Connect PACCAR ESA and activate the DEF supply pump to monitor live pressure output
- Inspect the DEF supply line routing for kinks, damage, or blockage between the tank module and the dosing unit
- Verify DEF quality and level are adequate
- If filter and lines are clear and pressure remains low, the supply pump module may require replacement
Can I Keep Driving?
The MX-13 can operate with this fault active initially, but the SCR system will not dose adequately, and the inducement sequence will begin. Repair the supply pressure fault before the inducement advances further.
Related Codes
Related Lookup Pages
Sources
- PACCAR / Kenworth / Peterbilt Service Support — MX Engine Resources PACCAR Inc. · oem · accessed 2026-06-11 · confidence medium
Source: PACCAR Inc., PACCAR / Kenworth / Peterbilt Service Support — MX Engine Resources. 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 SPN 3361 FMI 1 on the PACCAR MX-13 indicate a dosing injector problem?
No. SPN 3361 FMI 1 is a supply pressure fault, meaning the pump is not generating adequate pressure in the DEF supply circuit. A dosing injector problem would more likely generate SPN 3480 (dosing injector circuit fault). The two faults can coexist, but they should be diagnosed separately. Supply pressure must be restored before the injector's function can be accurately evaluated.
How is the DEF supply pump tested on the PACCAR MX-13?
PACCAR ESA includes a DEF pump activation test that commands the pump to run while displaying supply pressure in real time. This test shows whether the pump can build adequate pressure without a restriction, and whether pressure holds when the pump stops. If the pump runs but pressure remains low with clear lines and filter, the pump motor or internal check valve may have failed.
Can the DEF supply filter on the PACCAR MX-13 be replaced separately from the module?
On many PACCAR MX-13 applications, the DEF filter is a replaceable cartridge within the DEF module. The service procedure for filter replacement is outlined in the PACCAR service documentation and requires careful handling to avoid introducing air or contamination into the system. PACCAR ESA may show a filter restriction indicator or pressure delta that can help confirm the filter is the source of the pressure fault.