Code Details
| Display code | SPN 3364 FMI 1 |
|---|---|
| SPN | 3364 |
| FMI | 1 |
| OEM code | None listed |
| Manufacturer | Cummins |
| System | Aftertreatment �?DEF quality |
| Component | DEF quality sensor / DEF tank / DEF fluid |
| Source address | Unknown or not applicable |
| Severity | high |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-12 |
Plain-English Meaning
The ISB 6.7 DEF quality sensor measures the concentration of urea in the DEF fluid. DEF must be 32.5% urea in deionized water (ISO 22241 specification) to work correctly in the SCR system. When the sensor reads a concentration below specification, SPN 3364 FMI 1 is set. In medium-duty Kenworth T270, Peterbilt 325, and Freightliner M2 trucks, this fault is often caused by refilling with diluted DEF, water that entered the tank, or DEF that was stored too long and degraded.
SPN 3364 FMI 1 is DEF Quality below the normal specification. The DEF quality sensor uses ultrasonic measurement to detect urea concentration. On Cummins ISB 6.7 CM2350, Insite displays the DEF quality sensor reading and can compare it to the specification threshold. FMI 1 indicates a valid below-normal reading �?the sensor is functional but reporting a low-quality fluid.
Common Symptoms
- DEF quality warning on the instrument cluster
- SCR efficiency fault (SPN 5246) may follow if poor DEF is being dosed
- Inducement sequence may be active or approaching
- Higher than normal exhaust NOx output
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- DEF diluted with water or other fluid during filling
- DEF degraded from prolonged storage in hot conditions (urea breaks down above 30°C)
- Incorrect fluid filled �?water or a non-DEF fluid in the DEF tank
- Water contamination from a loose DEF filler cap
- DEF quality sensor fault (less common)
First Checks
- Test a sample of the DEF with a refractometer �?correct DEF reads 31.8�?3.2% urea concentration
- If DEF is out of specification, drain the tank completely and refill with fresh ISO 22241 DEF
- Connect Cummins Insite and run the DEF quality sensor test to confirm the sensor is reading correctly after a refill
- Inspect the DEF filler cap for seal damage that may have allowed water contamination
- Clear the fault with Insite after the tank is refilled and allow a drive cycle to confirm resolution
Can I Keep Driving?
Continued operation with poor DEF quality will degrade SCR performance and trigger the inducement sequence. Drain and replace DEF as soon as possible.
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
Can the ISB 6.7 be damaged by running poor-quality DEF?
Poor DEF quality reduces SCR conversion efficiency, which allows higher NOx to pass through the aftertreatment. Prolonged operation with significantly out-of-spec DEF can contribute to SCR catalyst surface contamination over time. The dosing injector may also accumulate crystalline deposits if urea concentration is excessively high (over-concentrated DEF).
Does the refractometer test work for DEF at any temperature?
Refractometers for DEF measure specific gravity or refractive index, which correlates to urea concentration. Most hand-held refractometers have a temperature compensation function or require the fluid to be tested at a consistent temperature (usually room temperature ~20°C). Freshly collected DEF from a hot tank or cold tank should be allowed to reach room temperature before testing for the most accurate result.
Is Cummins Insite required to clear SPN 3364 on the ISB 6.7 after replacing DEF?
After replacing the DEF with quality fluid, the code may clear on its own during a full drive cycle as the new DEF is drawn through the system and the sensor confirms correct quality. However, if the code does not clear within a drive cycle, Insite can force a clear. The inducement counter may also need to be reviewed in Insite after a confirmed repair.