Code Details
| Display code | SPN 3364 FMI 1 |
|---|---|
| SPN | 3364 |
| FMI | 1 |
| OEM code | None listed |
| Manufacturer | Cummins |
| System | Aftertreatment �?DEF system |
| Component | DEF quality / concentration sensor |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The ISB 6.7 is widely used in medium-duty trucks, school buses, and transit buses. Like its heavier siblings, it relies on DEF at approximately 32.5% urea concentration to operate the SCR system. When the quality sensor reads below that target �?from wrong fluid, dilution, or DEF aging �?the ECM logs this code and begins the inducement countdown that eventually derates the engine.
SPN 3364 is Aftertreatment 1 DEF Concentration in J1939. FMI 1 indicates a valid reading below the normal range. On Cummins CM2350B ECMs used in the ISB 6.7, the DEF quality sensor monitors urea concentration in real time. When the reading falls below the acceptable floor, the ECM activates the SCR inducement logic. The ISB 6.7 inducement sequence follows the same progressive stages as heavier Cummins platforms: warning lamp, then torque derate, then speed limit. In bus applications, operators may have telematics alerts configured for DEF system faults.
Common Symptoms
- Amber check engine or DEF warning lamp active
- DEF quality message on the driver display or fleet management system
- Inducement distance counter running in telematics
- No immediate power loss at fault set but derate risk increases with accumulated distance
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF tank filled with water or wrong fluid
- DEF diluted by water ingestion through a leaking cap or improperly sealed system
- DEF aged beyond usable concentration from heat cycling or improper storage
- DEF quality sensor fault after extended high-mileage operation
- Wiring harness fault affecting sensor signal accuracy
First Checks
- Sample the DEF from the tank into a clean container and visually check for cloudiness, discoloration, or unusual odor
- Connect Cummins Insite and check the DEF quality sensor live data and inducement counter distance
- Inspect the DEF filler cap and tank for cracks or damaged seals
- If DEF quality is good but the sensor still reads low, inspect the wiring harness and connector at the quality sensor for corrosion or damage
Can I Keep Driving?
The ISB 6.7 can continue operating in a warning state, but the inducement timer advances with every mile. In bus or fleet applications where vehicles are on fixed routes, address this promptly to avoid an in-service derate.
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
Is the DEF inducement process on the ISB 6.7 the same as on the ISX15?
The general inducement logic is similar across Cummins CM2350 platforms �?progressive warning, then derate, then speed limit �?but the specific distance thresholds can vary by calibration. Cummins Insite shows the current counter value for the specific engine so you know exactly how far into the sequence the system has progressed.
Can SPN 3364 FMI 1 cause a school bus or transit bus to be put out of service?
If the fault reaches the derate stage, the vehicle may not be able to maintain speed or complete its route. Many fleet operators and school districts have maintenance protocols that require DEF system faults to be addressed before the vehicle returns to service, regardless of whether a derate has activated.
What happens if I keep adding fresh DEF but the fault keeps returning on the ISB 6.7?
Recurring faults after adding fresh DEF suggest either a DEF quality sensor fault, a contamination source that keeps degrading new DEF, or a wiring issue. Use Insite to monitor the quality sensor output with a fresh tank; persistent low readings with verified good DEF indicate a sensor or circuit problem.