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SPN 3364 FMI 1

Aftertreatment 1 DEF Quality — Value too low (most severe).
The reading is valid but far below the normal operating range, at the most serious 'too low' level.

What the SPN covers: Whether the fluid in the DEF tank has the right urea concentration. More on SPN 3364.

What FMI 1 usually points to: A real under-condition such as very low oil pressure or low fuel pressure, or a sensor that has drifted low. More on FMI 1.

Severity

MEDIUM — fix soon

Poor DEF quality causes inducement derates. Drain and refill with fresh certified DEF.

Likely causes

  1. Diluted or contaminated DEF (water, coolant, wrong fluid)
  2. Old DEF that has degraded in heat
  3. Faulty quality sensor

Checks & fixes (easiest first)

  1. Test DEF with a refractometer
  2. Drain, flush and refill with fresh DEF
  3. Inspect the tank sender for contamination

FAQ

What does SPN 3364 FMI 1 mean?

Aftertreatment 1 DEF Quality — value too low (most severe). The reading is valid but far below the normal operating range, at the most serious 'too low' level.

Is SPN 3364 FMI 1 serious?

Poor DEF quality causes inducement derates. Drain and refill with fresh certified DEF.

What should I check first for SPN 3364 FMI 1?

Test DEF with a refractometer; Drain, flush and refill with fresh DEF; Inspect the tank sender for contamination.