Code Details
| Display code | SPN 3361 FMI 1 |
|---|---|
| SPN | 3361 |
| FMI | 1 |
| OEM code | None listed |
| Manufacturer | Detroit Diesel |
| System | Aftertreatment �?DEF supply system |
| Component | DEF supply pump / DEF lines / filter / DEF tank module |
| 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 system on the DD15 uses a pump to draw DEF from the tank and deliver it at pressure to the dosing injector. When supply pressure falls below the normal threshold, the dosing system cannot deliver adequate DEF, reducing SCR efficiency. The most common causes are a clogged DEF filter, a restricted supply line, frozen DEF in cold weather, or a worn pump. On Freightliner Cascadia and Western Star trucks with the DD15, the NOx sensor upstream of the SCR catalyst provides the reference measurement for efficiency calculation. DiagnosticLink monitors both NOx sensors and can run a NOx sensor diagnostic to test sensor signal quality and circuit integrity. A failed upstream NOx sensor that reads stuck at zero makes the SCR system appear to have perfect efficiency — because the calculated efficiency numerator is zero. This failure mode does not trigger SPN 5246 but may allow a genuine catalyst problem to go undetected. Regular NOx sensor signal quality checks in DiagnosticLink are part of the DD15 aftertreatment preventive maintenance program.
SPN 3361 FMI 1 is Aftertreatment DEF Dosing Unit Pressure below normal range. On Detroit DD15 with the Detroit aftertreatment package, DiagnosticLink can command the DEF supply pump and monitor commanded versus actual supply pressure. The DEF pump on most DD15 configurations is integrated into a module in the DEF tank that also contains the level sensor, quality sensor, and heating element.
Common Symptoms
- DEF system warning on the instrument cluster
- SCR efficiency fault may accompany this code
- In very cold weather, may appear after initial startup before the DEF system fully thaws
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Plugged DEF supply filter in the tank module
- DEF supply line restricted or kinked
- DEF partially frozen in cold weather
- DEF supply pump worn or failing
- DEF pressure sensor fault
First Checks
- Connect DiagnosticLink and command the DEF pump �?monitor commanded vs. actual supply pressure
- Replace the DEF supply filter if overdue or if contaminated DEF has been used
- In cold weather, allow the DEF system warm-up cycle to fully complete before evaluating this fault
- Check DEF level and confirm DEF quality meets ISO 22241
- Connect DiagnosticLink and run the NOx sensor diagnostic for both upstream and downstream sensors
- Compare NOx sensor readings at various load conditions to confirm both sensors are tracking exhaust gas changes
Can I Keep Driving?
Driveable but inadequate DEF supply degrades SCR efficiency and may trigger the emissions inducement sequence.
Related Codes
Related Lookup Pages
Sources
- Detroit Diesel Service and Diagnostic Resources — Public Reference Detroit Diesel (Daimler Truck North America) · oem · accessed 2026-06-11 · confidence medium
Source: Detroit Diesel (Daimler Truck North America), Detroit Diesel Service and Diagnostic Resources — Public Reference. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - DiagnosticLink Detroit Diesel Corporation · oem · accessed 2026-05-05 · confidence medium
Source: Detroit Diesel Corporation, DiagnosticLink. 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
Is the DEF tank module on the Detroit DD15 the same as on the DD13?
The DD15 and DD13 both use Detroit's integrated DEF tank module design that combines the pump, heater, level sensor, and quality sensor. The specific module part number may differ between DD13 and DD15 due to tank size and flow rate differences, but the diagnostic approach is the same. DiagnosticLink identifies the module and can test each component individually.
Can the DEF filter on the DD15 tank module be replaced separately or must the whole module be replaced?
On most Detroit DD15 DEF tank module designs, the filter element is a replaceable serviceable component. This is important because replacing just the filter is far less expensive than replacing the full module. However, if DiagnosticLink confirms the pump is not achieving commanded pressure even with a new filter and clear lines, the pump or module assembly requires replacement.
What DEF quality specification does the Detroit DD15 require?
The Detroit DD15 requires DEF meeting the ISO 22241 standard (also sold in North America under the trademarked name DEF or AdBlue in Europe). ISO 22241 DEF is 32.5% urea concentration in deionized water. Using non-compliant DEF or diluted DEF can cause SCR efficiency faults and crystal contamination in the supply system. The DD15 DEF quality sensor detects concentration and sets a separate fault if quality is out of range.