If your diesel truck is showing SPN 3364 FMI 9, you’re dealing with one of the most common and confusing DEF-related codes. This fault usually triggers DEF warnings, failed regens, and can eventually lead to a 5 MPH derate if ignored.
Below is the complete explanation, causes, and step-by-step repair process.
🚨 What SPN 3364 FMI 9 Means
SPN 3364 = DEF Quality (Urea Concentration)
FMI 9 = Abnormal Update Rate / Data Not Plausible
In plain English:
👉 Your truck does not trust the DEF quality sensor reading.
The ECM thinks the DEF is wrong, contaminated, or the sensor is lying.
⚠️ Symptoms of SPN 3364 FMI 9
You may notice:
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DEF light on
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Check Engine + MIL
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Countdown to 5 MPH derate
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Failed stationary regen
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Truck asking for DEF refill even with a full tank
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DEF pump running excessively
This is an emissions-related code — it will not clear on its own.
🔍 Most Common Causes of SPN 3364 FMI 9
✔️ 1. Contaminated DEF
DEF older than 6 months
DEF stored in heat
DEF mixed with water
Cheap gas station DEF
✔️ 2. Bad DEF Quality Sensor (in the DEF header)
Very common on Freightliner, International, Paccar, Volvo, and Ram 5500.
✔️ 3. DEF tank heater or temp sensor failure
✔️ 4. DEF crystallization around the head
✔️ 5. Broken wiring or corroded connectors
🛠️ How to Diagnose & Fix SPN 3364 FMI 9
✅ Step 1: Read the Code With a Diesel Laptop
Use:
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JPRO
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Cummins Insite
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Detroit DDDL
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Diesel Tech / Universal Diag
Scan for active codes related to:
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NOx sensors
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DEF dosing
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DEF pump
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SCR efficiency
These codes often occur together.
✅ Step 2: Check the DEF
Before changing parts, check your DEF:
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Is it clear?
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Does it smell like ammonia?
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Does it show crystallization around the tank cap?
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How old is it?
👉 If it’s cloudy, yellow, or watery → drain and refill with fresh DEF.
✅ Step 3: Inspect the DEF Header (Quality Sensor)
A failing DEF header triggers:
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SPN 3364
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SPN 1761
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SPN 3031
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SPN 3032
Look for:
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Cracked housing
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Water intrusion
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Crystallized buildup
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Loose connectors
If the header has visible damage → replace it.
✅ Step 4: Test NOx Sensors
Even if the fault points to the DEF system, a bad NOx inlet sensor can falsely trigger SPN 3364.
Check:
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SPN 4094
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SPN 4334
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SPN 3216
If any of these are active → replace the failing sensor first.
✅ Step 5: Run a DEF Quality or SCR Test
Using your diesel laptop:
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Open Aftertreatment diagnostics
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Select DEF Quality Test
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Let the test run 5–20 minutes
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Review sensor values
If the sensor fails the test → replace the DEF header.
✅ Step 6: Perform a Forced Regen
After repairs:
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Initiate a forced regen
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Monitor NOx conversion efficiency
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Make sure temps rise properly
A failed regen = underlying SCR issues still exist.
✅ Step 7: Clear the Code & Reset Derate Timers
Only after:
✔️ Fresh DEF
✔️ Sensor tests
✔️ Completed regen
Then use your diesel laptop to:
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Clear SPN 3364
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Reset inducement/derate timers
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Cycle ignition
🔧 Tools You Need to Fix SPN 3364 FMI 9
✔️ RP1210 Diagnostic Adapter
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Nexiq USB-Link 3
✔️ Diesel Diagnostic Laptop
Preloaded with:
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JPRO
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Diesel Tech
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Detroit DDDL
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Cummins Insite alternative
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Forced regen utilities
👉 DieselScanners kits come fully set up and activated.
💡 Pro Tip
Do NOT clear SPN 3364 without fixing the root cause.
It will return instantly and continue counting down to derate.
🚛 Final Summary
SPN 3364 FMI 9 is a DEF quality-related fault that commonly leads to:
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Failed regens
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5 MPH derate
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Incorrect DEF readings
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SCR system faults
Fixing it requires:
✔️ Scanning the truck
✔️ Testing DEF
✔️ Checking the DEF header
✔️ Inspecting NOx sensors
✔️ Performing a forced regen
Once everything is resolved, you can clear the code and reset the derate.