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SPN 3364 FMI 18

Aftertreatment 1 DEF Quality — Value too low (moderately severe).
The reading is below normal at a middle warning level — more serious than FMI 17, less than FMI 1.

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

What FMI 18 usually points to: A developing under-condition that often precedes a derate. More on FMI 18.

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 18 mean?

Aftertreatment 1 DEF Quality — value too low (moderately severe). The reading is below normal at a middle warning level — more serious than FMI 17, less than FMI 1.

Is SPN 3364 FMI 18 serious?

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

What should I check first for SPN 3364 FMI 18?

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