SPN 3364: Aftertreatment DEF Quality
What SPN 3364 Is
SPN 3364 is Aftertreatment 1 DEF tank quality / concentration — usually the quality sensor in the tank header. It is not tank level (1761) and not outlet NOx (3226). Cummins often shows this family as FC 3868 (DEF quality).
The ECM expects ~32.5% urea. Diesel, water, old fluid, or a failed quality sensor all set 3364. Contamination also poisons SCR and shows up later as 4364.
Common FMI Combinations
| FMI | Meaning | Start here |
|---|---|---|
| 1 / 18 | Quality below normal | Refractometer, then drain/flush |
| 2 | Erratic | Header connector, sensor, aeration |
| 3 / 4 / 5 | Circuit | Header harness |
| 14 | Special instructions | OEM flush procedure — follow it |
Diagnostic Order
- Refractometer on a sample from the tank, not from a jug on the shelf.
- If concentration is wrong, drain, flush the tank and lines, refill with API DEF. Replacing the quality sensor on contaminated fluid is a repeat job.
- If concentration is good and FMI is circuit/erratic, treat it like any header sensor: connector first, then sensor.
- Look for companion 4374 (pump) and 4364 (efficiency).
Remote diagnostics: Nexiq USB-Link 3 + diagnostic laptop, live DEF concentration, tank temp, and pump pressure on the same screen.
Frequently Asked Questions
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