SPN 3364 FMI 2
Derate riskDEF Quality / Concentration — Data Erratic or Incorrect
SAE J1939 · heavy duty · Freightliner, Volvo Trucks, PACCAR (Kenworth / Peterbilt), International, Mack, Cummins, Western Star
Power loss coming
The truck thinks your DEF is the wrong concentration or the quality sensor is giving impossible readings. It is usually the DEF or the sensor, not the engine. Fix it before the inducement clock runs out.
DerateTODO(verify): confirm the inducement schedule for each OEM. Most 2013+ SCR systems escalate through a torque reduction to a 5 mph speed limit if the fault stays active across successive drive cycles and refills.
DrivingYes, for now — but you are on a clock. This fault starts an inducement sequence, and once it escalates you will be limited to a speed that makes the interstate genuinely dangerous. Get the fluid tested at the next stop rather than at the next scheduled service.
Typical repair cost
- Parts
- $810–$2,350
- Labor
- 1–3 hr × $125–$210/hr = $125–$630
Provisional figures — pending verification against our receipt dataset.
Labor rate: provisional national placeholder — TODO(verify), not yet from our collected dataset. How we build these ranges.
Need it fixed
Phone-verified shops, with the date we last called each one.
What SPN 3364 FMI 2 means
SPN 3364
SPN 3364 is the J1939 parameter for aftertreatment 1 diesel exhaust fluid quality — the measured urea concentration of the fluid in the tank. Correct DEF is 32.5% urea in deionised water; the quality sensor reports the concentration it measures so the SCR system can decide whether dosing will actually work.
FMI 2
FMI 2 means data erratic, intermittent, or incorrect. The ECM is receiving a signal from the quality sensor, but the value is jumping around or falls outside any physically plausible concentration. This is different from FMI 3/4 (voltage high/low, an electrical fault) and different from FMI 1 (concentration genuinely low but stable).
Read together, SPN 3364 FMI 2 is the ECM saying: def quality / concentration — data erratic or incorrect. The SPN identifies which parameter, the FMI identifies how it failed. Two codes sharing an SPN but carrying different FMIs are different faults with different repairs, which is why the SPN alone is never enough.
Common causes
- Wrong fluid in the DEF tank — water, diesel, washer fluid, or a non-API-certified urea mixThe single most common cause reported in service literature. TODO(verify): frequency from our receipt dataset once n >= 5.
- Degraded DEF — heat-aged, frozen and thawed repeatedly, or past its shelf lifeUrea breaks down above roughly 86°F over months. TODO(verify): exact degradation curve and OEM shelf-life guidance.
- Failed DEF quality sensor or its wiring / connector corrosion
- Contaminated tank after a bad fill — the tank holds residue even after a refill
- Software calibration out of date on the aftertreatment controller
No frequency data yet. We have fewer than 5 verified repair receipts for SPN 3364 FMI 2, so we are not going to print percentages. Causes above are listed in no particular order. Had this fixed? Tell us what it turned out to be.
Technician note pending
A credentialed technician annotation for SPN 3364 FMI 2 has not been commissioned yet. Until it is, this page stays a draft and is excluded from our sitemap. We do not publish invented first-hand experience. How we verify content.
How a technician diagnoses it
- 1
Before touching anything electrical, test the fluid in the tank with a refractometer. Correct DEF reads 32.5% urea, with an acceptable band per ISO 22241.
Tool: DEF refractometer · Spec: 32.5% urea ±1.5% (TODO(verify): confirm exact acceptance band against ISO 22241-1)
- 2
Smell and look at the fluid. DEF is clear and faintly ammoniac. Cloudy, coloured, or diesel-smelling fluid is a drain-and-flush, not a sensor job.
- 3
Pull freeze-frame data and check whether the fault set immediately after a DEF fill. A fault that appears within a tank of a specific truck stop is a fluid problem until proven otherwise.
- 4
Inspect the quality sensor connector for green corrosion, backed-out pins, and chafed harness where it passes the frame rail.
Tool: Inspection light
- 5
Check the sensor's supply voltage and ground reference against the OEM spec with the key on, engine off.
Tool: Digital multimeter · Spec: TODO(verify): supply voltage and ground drop spec per OEM
- 6
Verify the aftertreatment controller is on the current calibration before condemning the sensor. Several OEMs have issued software updates addressing false quality faults.
Tool: OEM service tool (DiagnosticLink, Premium Tech Tool, ServiceRanger, Diamond Logic Builder)
- 7
If the fluid tests good and the harness is clean, replace the quality sensor and clear. Confirm the fault does not return over a full drive cycle before releasing the truck.
Parts and labour
| Part | Part number | Parts cost |
|---|---|---|
| DEF quality / concentration sensor | TODO(verify) | $220–$650provisional |
| DEF tank drain and flush, plus refill with certified fluid | TODO(verify) | $90–$300provisional |
| DEF header / sender unit assembly (where the sensor is not serviced separately) | TODO(verify) | $500–$1,400provisional |
| Labour | 1–3 hr | $125–$630 |
Labour priced at provisional national placeholder — TODO(verify), not yet from our collected dataset. How we build these numbers.
Can I keep driving?
Yes, for now — but you are on a clock. This fault starts an inducement sequence, and once it escalates you will be limited to a speed that makes the interstate genuinely dangerous. Get the fluid tested at the next stop rather than at the next scheduled service.
TODO(verify): confirm the inducement schedule for each OEM. Most 2013+ SCR systems escalate through a torque reduction to a 5 mph speed limit if the fault stays active across successive drive cycles and refills.
Find a shop that handles this
We call every shop before it appears here, and publish the date.
Sample records only — seeded to demonstrate the directory. None of these numbers has been called.
Memphis, TN · 24/7 · mobile service
Indianapolis, IN · Mon-Fri 6am-6pm · mobile service
Laredo, TX · Mon-Sat 6am-9pm · mobile service
Phoenix, AZ · 24/7 · mobile service
Codes that usually show up with this one
- SPN 5246 FMI 0SCR Operator Inducement — Most Severe Level ActiveOut of service
- SPN 4364 FMI 18SCR Conversion Efficiency — Below Normal, Moderately SevereDerate risk
- SPN 3216 FMI 16SCR Inlet NOx Sensor — Reading Above Normal RangeCaution
- SPN 1569 FMI 31Engine Protection Torque Derate — Condition ExistsDerate risk
Had this repaired? Tell us what you paid.
Every price range on this site comes from real invoices. Two minutes of your time makes the next driver's estimate better — and you get to see the aggregate for SPN 3364 FMI 2 once it publishes.
No account. No email required. We publish aggregates only, never your invoice.
Submit a repair receipt →