SPN 3364 FMI 18
CAUTION
DEF quality
The system detected DEF that is out of specification.
SPN 3364 FMI 18: The system detected DEF that is out of specification. Caution: you can keep driving carefully, but the engine may derate; get it addressed soon.
Electrical · SCR / DEF
Can I keep driving?
Caution: you can keep driving carefully, but the engine may derate; get it addressed soon.
What it means
The quality sensor reports urea concentration off the nominal 32.5%. Left uncorrected it escalates to inducement and derate. Check the fluid first, not the sensor.
Most likely causes
DEF diluted with water or topped off with the wrong fluid
Very likely
Expired DEF or DEF stored hot
Likely
Faulty DEF quality sensor
Likely
Corroded sensor harness or connector
Less likely
Tests, cheapest and most decisive first
1. Measure concentration with a refractometer
Expected result: 32.5% urea
Tool: Refractometer
2. Inspect the quality sensor connector
Expected result: Clean, tight pins with no corrosion
Part usually replaced
DEF tank drain & refill (contaminated fluid)
$80–$350 (aftermarket to dealer)
Labor: 1–2 h
USD, part only. Labor is billed at the shop's hourly rate.
Data: DERATE verified base, updated October 2026
Related codes
Read it off your own dash
Take one photo and DERATE tells you whether you can keep driving.
Ask DERATE
DERATE assists. It does not replace inspection by a qualified technician.