Has your equipment thrown fault code SPN 3364 FMI 2? This guide explains what the code means, its common reasons, and simple step-by-step fixes to get your machine running reliably again.
What Does SPN 3364 FMI 2 Mean?
The SPN 3364 FMI 2 means a problem with the Diesel Exhaust Fluid (DEF) quality sensor. The system detects that the DEF in the tank does not meet the required specs for the Selective Catalytic Reduction (SCR) system to work properly.
- SPN 3364 stands for Suspect Parameter Number, and it identifies the faulty part — in this case, the aftertreatment DEF quality sensor.
- FMI 2 means Failure Mode Identifier, which tells you the fault type. FMI 2 translates to erratic, intermittent or incorrect data. Put simply, the sensor sends unstable or out-of-range signals, so the engine control module (ECM) thinks the DEF quality is bad.
This code pops up often on modern diesel trucks, including Detroit Diesel DD13 and DD15 engines used in many Freightliner trucks. It can also show up on PACCAR and Cummins engines, where it sometimes signals an electrical problem within the DEF dosing unit.
Common Symptoms of Code SPN 3364 FMI 2
When this fault code triggers, you’ll probably notice several clear symptoms.
- Check Engine Light (MIL): The Malfunction Indicator Lamp lights up on your dash.
- Engine Derate: The ECM cuts engine power to protect the delicate, costly aftertreatment system from damage. This usually limits the truck’s speed, sometimes down to just 5 mph.
- Dashboard Alerts: Messages such as “Poor DEF Quality” or “Check DEF” will show on the instrument panel.
- Blinking DEF Light: The DEF indicator light may flash to warn you of this serious fault.
Main Causes of SPN 3364 FMI 2 Error
While this code links to the DEF quality sensor, the real problem may not be the sensor at all. Below are the most common causes, starting with the most likely one.
Poor DEF Quality
This is by far the top reason for the SPN 3364 FMI 2 fault. DEF needs to be an exact mix: 32.5% high-purity urea and 67.5% deionized water. Any mismatch will trigger the code.
- Contamination: Even tiny amounts of other liquids such as diesel, oil, coolant or regular tap water can contaminate the tank and set off the fault.
- Degraded Fluid: DEF does not last forever. Direct sunlight or extreme heat and cold can make it break down and no longer meet standards.
- Bad Batches: DEF bought from stores with slow stock turnover or from bulk pumps can be risky. The fluid might be old, contaminated or stored incorrectly. Many drivers fixed this problem just by switching from bulk DEF to sealed, boxed DEF from a reliable brand.
Clogged DEF Dosing Valve or Injector
The DEF doser sprays a fine mist of fluid into the exhaust. Over time, DEF crystals can build up and clog this valve. A poor spray pattern stops DEF from mixing well with exhaust gas, which hurts NOx conversion. The system may mistake this issue for bad DEF quality.
Faulty DEF Quality Sensor
The sensor itself can go bad. This complex part sits inside the DEF tank. Like other electronic components, it can wear out or malfunction, sending unstable readings to the ECM even when the DEF is fine.
Electrical Issues
As seen on some PACCAR and Cummins systems, this fault can come from electrical problems. That includes damaged wiring, corroded connectors, or an internal short inside the DEF dosing unit or quality sensor.
Failed NOx Sensors
The SCR system uses inlet and outlet NOx sensors to check how well it works. If one sensor fails, it may send wrong NOx reduction readings. The ECM can misread this data as poor DEF quality and throw the 3364 code, even when the real problem is a downstream sensor.
How to Fix SPN 3364 FMI 2?
Work through these troubleshooting steps in order, starting with the easiest and most likely fixes.
Step 1: Drain and Replace the DEF
Bad DEF is the most common cause, so always start here.
- Take a look at the fluid inside the tank. It should be completely clear. If it looks cloudy, discolored, or has particles in it, the DEF is contaminated.
- Drain the whole tank. Some trucks come with a drain plug. If yours doesn’t, use a clean hose to siphon out the fluid safely.
- Fill the tank with fresh, high-quality DEF from sealed containers bought from a trusted seller.
- Once refilled, cycle the ignition. Turn the key to the “on” position for two minutes, then switch it off for two minutes. Repeat this a few times to push the new fluid through the lines and sensor.
Step 2: Check for Other Fault Codes
SPN 3364 FMI 2 often shows up with other codes. You may see SPN 4364 FMI 18 (low SCR NOx conversion efficiency) or SPN 5246 (aftertreatment derate). These related codes help confirm the problem is with SCR performance instead of just a random sensor error.
Step 3: Inspect and Clean the DEF Dosing Unit
If fresh DEF doesn’t clear the code, check the dosing injector next.
- Carefully take the injector off the exhaust pipe. Check the nozzle for white, crusty DEF crystal buildup.
- Softly clean the nozzle with a soft brush and warm water to remove all crystals. Don’t use strong chemicals or metal tools, as these can damage the part.
- Put the injector back in place and see if the fault goes away.
Step 4: Test Sensors and Wiring
If the issue remains, move on to more advanced checks.
- DEF Quality Sensor: A mechanic can use diagnostic software to read the sensor’s live output. If the readings jump around or don’t change when using known good DEF, the sensor is probably bad and needs replacement.
- NOx Sensors: Check readings from the inlet and outlet NOx sensors during a forced regen or while driving. You should see NOx levels drop noticeably after the SCR catalyst. If a sensor shows no value or a fixed unchanging number, it is faulty.
- Wiring Harness: Check all wires and connectors running between the ECM, DEF tank assembly and dosing valve. Watch for corrosion, rubbed wiring or loose pins.
Wrapping Up
SPN 3364 FMI 2 is usually easy to fix if you check it step by step. In many cases, the problem comes from bad or contaminated DEF. Use fresh DEF from a trusted source and store it properly. If a part has failed, check FridayParts for compatible replacement parts that are affordable and reliable.
