SPN 3364 FMI 0/1 — DEF Quality Above / Below Normal (Concentration Rationality)
DEF quality high or low codes begin with a refractometer test of the fluid in the tank. Correct the fluid before condemning the sensor.
Fault Overview
FMI 0 or 1 on SPN 3364 indicates measured DEF concentration is above or below the acceptable window (typically outside ~28–36% or platform-specific limits), or the sensor reports values that fail rationality checks against expected urea concentration.
Engine & Platform Coverage
Symptoms
- •DEF quality low
- •DEF quality high
- •Poor DEF quality
- •SPN 3364 concentration
- •Contaminated DEF
- •Inducement DEF quality
Possible Causes
- 1.Contaminated or diluted DEF (water, diesel, coolant)
- 2.Incorrect fluid added
- 3.Failed quality sensor
- 4.Crystallization affecting the sensor
- 5.Frozen or partially thawed DEF
- 6.Wiring issues producing out-of-range signals
Wiring & Circuit Notes
Check header connector and circuits for opens, shorts, or contamination that could skew the reading. Inspect for crystallization at the sensor area.
DVOM Specs & Sensor Tests
Target concentration 32.5% urea. Use a digital refractometer on a tank sample. Compare sensor-reported value to the physical measurement. Sensor signal should be stable and within calibrated range when fluid is good.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Step 1 — Obtain a Clear Sample from the Tank: Obtain a clear sample from the tank. The majority of quality codes are caused by actual contaminated or incorrect fluid rather than sensor failure.
- 2Step 2 — Measure Concentration with a Calibrated Refractometer: Measure concentration with a calibrated refractometer. Target is 32.5% urea. Values outside ~28–36% confirm contaminated or incorrect fluid.
- 3Step 3 — Drain and Refill with API-Certified DEF if Out of Range: If outside 32.5% ± tolerance, drain and refill with API-certified DEF. Clear codes and monitor before any sensor work.
- 4Step 4 — If Fluid is Good, Check Circuits and Reference Voltage: If fluid is good but sensor still reports out-of-range, check circuits and reference voltage at the header connector.
- 5Step 5 — Inspect Sensor for Contamination or Crystals: Inspect sensor for contamination or crystals. Clean thoroughly and retest.
- 6Step 6 — Replace Header/Sensor Only After Fluid and Circuit Proof: Replace header/sensor only after fluid and circuit proof. Always test the fluid before replacing the header.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Typical Repair Costs
Drain and refill with correct DEF $30–100; clean sensor/connector $50–150; repair wiring $100–300; replace header if sensor is failed $200–600. Total $30–650.
Start Diagnosing This Fault Now
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Summary
Use the AI Diesel Tech diagnostic engine to run this code through a structured, evidence-gated sequence and get a confirmed root cause before any parts are replaced.