What SPN 3364 FMI 2 means
SPN 3364 FMI 2: Aftertreatment DEF quality — data erratic, intermittent or incorrect.
Applies to: All Cummins SCR engines with DEF quality sensor, 2013–2026.
Severity: High — starts the SCR inducement countdown.
Symptoms you'll see
- DEF warning lamp
- Derate countdown message
- Code may appear after refilling DEF from an unfamiliar source
Likely causes (ranked by probability)
- Contaminated or diluted DEF (~50%) — roadside drums and old stock are the usual suspects
- Failed DEF quality sensor (~30%)
- Crystallized DEF contaminating the sensor (~15%)
- Wrong fluid in the DEF tank entirely — it happens (~5%)
Diagnostic steps
- Test the DEF: refractometer or test strips — 32.5% urea ± 1.5% is spec.
- If DEF fails: drain, flush with distilled water, refill with fresh sealed DEF, clear codes, run SCR test.
- If DEF passes: inspect the quality sensor for crystallization; clean or replace.
- Verify with live data — quality percentage should stabilize near 32.5%.
Tools that read, clear and verify this code
DEF test strips cost $10 and answer half these cases before any scanner is needed; verification via INSITE or any CV scanner. For the full per-platform picture see the Cummins ISX fault codes page and our SCR/DEF service guide.
Frequently Asked Questions
Can I drive with SPN 3364 FMI 2 active?
Severity on this code: High — starts the SCR inducement countdown. If a derate countdown is armed, use the remaining runtime to reach a shop — not to finish the week. Verify the root cause with the diagnostic steps above rather than clearing and hoping.
Will clearing SPN 3364 FMI 2 without repairs work?
The code will return — usually within one drive cycle — and on aftertreatment-related faults the inducement countdown often shortens each time. Repair first, verify with the guided test, then clear.
Which scanner reads SPN 3364 FMI 2?
DEF test strips cost $10 and answer half these cases before any scanner is needed; verification via INSITE or any CV scanner.