If your diesel truck is showing SPN 3364 FMI 2, you’re dealing with a DEF quality fault where the ECM is receiving erratic or incorrect DEF concentration data. This code almost always leads to:
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Failed regens
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DEF warnings
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SCR efficiency faults
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SPN 5246 derate countdown
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5 MPH limp mode
This guide covers what the code means, why it happens, and exactly how to fix it using your diesel laptop.
🚨 What SPN 3364 FMI 2 Means
SPN 3364 = DEF Quality (Urea Concentration Reading)
FMI 2 = Data Erratic / Intermittent / Incorrect
In simple terms:
👉 The DEF quality sensor is sending unstable or impossible readings.
This is different from FMI 9 (abnormal update rate).
FMI 2 means the sensor IS reporting, but the data is inconsistent, spiking, or illogical.
⚠️ Common Symptoms
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DEF light on
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Check Engine + MIL
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Stabilitrak / Emissions warnings
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“Service DEF System”
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Frequent regen failures
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Truck stuck in low-power mode
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Countdown to speed/torque derate
🔍 Most Common Causes of SPN 3364 FMI 2
✔️ 1. Failing DEF Quality Sensor (Most Common)
Located in the DEF header.
Internal circuit becomes unstable → erratic readings.
✔️ 2. DEF Contamination
Water contamination
Old DEF (6+ months)
Improper storage
Mixed with diesel (yes, this happens)
✔️ 3. Corroded Wiring or Loose Connector Pins
Moisture in the DEF tank area
Bad ground
Loose terminals
✔️ 4. Low DEF Temperature or Heater Issues
If DEF is cold → bad readings
If tank heater failed → sensor readings fluctuate
✔️ 5. NOx Sensor Failure Triggering False DEF Issues
Bad NOx sensors can create incorrect DEF quality logic.
🛠️ How to Diagnose & Fix SPN 3364 FMI 2 (Step-by-Step)
✅ Step 1: Scan the Truck with Your Diesel Laptop
Use:
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JPRO
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Detroit DDDL
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Diesel Tech / Universal Diag
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Cummins Insite
Check for related faults:
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SPN 4094 (DEF dosing)
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SPN 4364 (SCR efficiency)
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SPN 3216 (NOx sensor)
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SPN 5246 (derate)
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SPN 3031/3032 (DEF header)
If ANY of these are active → they must be fixed first.
✅ Step 2: Inspect & Test the DEF
Look for:
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Cloudy DEF
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Yellow tint
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Floating particles
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Strong ammonia smell (bad)
If in doubt → drain and refill with fresh DEF.
✅ Step 3: Inspect the DEF Header and Wiring
Check:
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Connector pins for corrosion
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Moisture or coolant intrusion
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Crystallization around the cap
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Physical cracking
If the DEF header is older than 3–5 years → replacement is recommended.
✅ Step 4: Run the DEF Quality Test
Using your diesel laptop:
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Open Aftertreatment → DEF Tests
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Select “DEF Quality Test” or “SCR Test”
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Monitor:
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Urea concentration (%)
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NOx conversion
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Temperature stability
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Erratic swings in the DEF quality reading confirm the fault.
✅ Step 5: Replace the DEF Header if Needed
If readings remain unstable:
✔ Replace DEF header
✔ Clear codes
✔ Reset learned aftertreatment data
✅ Step 6: Perform a Forced Regen
This validates:
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SCR efficiency
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NOx conversion
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DEF dosing rate
If regen fails → further NOx or SCR work is required.
✅ Step 7: Clear Codes & Reset Derate Timers
After repairs:
✔ Clear SPN 3364
✔ Clear SCR inducement
✔ Cycle ignition
✔ Road test
💻 Tools Needed to Fix SPN 3364 FMI 2
✔ RP1210 Adapter
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Nexiq USB-Link 3
✔ Diesel Laptop Software
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Diesel Tech
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JPRO
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DDDL
DieselScanners kits come fully activated and preinstalled.
💡 Pro Tip
Erratic DEF quality readings often point to wiring failure, NOT the DEF itself.
If the quality jumps from:
32% → 18% → 57% → 22%
…your DEF header is failing.
🚛 Final Summary
SPN 3364 FMI 2 means:
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DEF quality readings are unstable
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DEF sensor (header) is failing
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Wiring or temperature issues may exist
Fixing requires:
✔ Scanning the truck
✔ Fresh DEF
✔ Checking header wiring
✔ Running DEF quality test
✔ Forced regen
✔ Clearing inducement timers