When SPN 4364 FMI 31 shows up on an off-road machine, it usually comes with the same pain points: derate, poor power, higher DEF use, forced regens, or a “won’t clear” fault that keeps returning. In this guide, we’ll explain what the code typically means on modern SCR-equipped heavy equipment, what actually causes it in the field, and a practical fix plan you can follow to get back to reliable work.
What is SPN 4364 FMI 31?
On many SCR (Selective Catalytic Reduction) aftertreatment systems used in off-road diesel equipment, SPN 4364 is commonly tied to SCR NOx conversion efficiency (in simple terms: the system isn’t reducing NOx the way the ECM expects). The ECM compares readings—mainly from upstream/downstream NOx sensors plus temperature/operating conditions—and decides whether the SCR is doing its job. When it isn’t, you can see inducements like reduced torque, limited speed, or a countdown strategy depending on the machine’s rules.
FMI 31 in the J1939 fault structure generally indicates “Condition Exists”—meaning the controller sees a real, active condition that meets the fault logic (not just an electrical short/open). That’s why this fault often points to performance issues (DEF dosing, leaks, catalyst efficiency) rather than a single clean “broken wire” situation—although wiring can still be the root cause if it corrupts sensor readings.
Typical symptoms of off-road machinery
- Engine derate / limp strategy during work
- Frequent regens or regens that won’t complete
- DEF consumptionis higher than normal (or sometimes oddly low)
- Fault clears briefly, then returns under load/temperature
What Causes SPN 4364 FMI 31 Code?
We’ve found SPN 4364 FMI 31 is usually triggered by one of two patterns:
- The SCR system truly isn’t converting NOx efficiently.
- The ECM thinks conversion is poor because sensor data is biased, delayed, or inconsistent.
Below are the most common cause groups for off-road equipment (loaders, excavators, tractors, harvesters, and other SCR-equipped machines).
1) NOx sensor problems (sensor aging, contamination, or heater issues)
NOx sensors work in a harsh environment. Heat cycles, soot, DEF crystals, or internal heater faults can cause readings to drift. If the downstream sensor reads too high (or the upstream reads too low), the ECM calculates low conversion efficiency—even if the catalyst is fine.
What to look for:
- Slow or “stuck” NOx readings in live data
- Upstream/downstream NOx values that don’t respond to load changes
- Sensor-related companion codes (heater, communication, rationality)
If replacement is needed, choose the correct fitment for your engine/equipment application. You can browse NOx sensors here. That catalog includes options covering common heavy equipment and diesel platforms (for example, listings for Komatsu SAA6D engines, CAT engines, John Deere skid steer applications, Kubota equipment, and more).
2) Exhaust leaks (especially between the DOC/DPF/SCR and at sensor bungs)
Small leaks upstream of the downstream NOx sensor can pull in fresh air and skew readings. Leaks near joints, clamps, bellows, or sensor ports can also change temperature and flow characteristics.
Common leak points:
- V-band clamps are not seated
- Flex pipe cracks
- Gaskets at aftertreatment connections
- Damaged sensor bungs/threads
This is where physical inspection matters. For replacement components, browse exhaust system parts that support emission/aftertreatment repairs.
3) DEF quality or DEF contamination
Poor DEF quality can mimic a “bad catalyst” because the dosing chemistry can’t work correctly. DEF should be the right concentration (commonly 32.5% urea) and uncontaminated.
Common real-world issues:
- DEF diluted with water
- Wrong fluid in the tank (cleaner, coolant, diesel contamination)
- Old DEF exposed to heat for long periods
- Crystal buildup from evaporation and poor cap sealing
4) DEF dosing system faults (injector, line, filter, pump, or deposits)
Even with good DEF, the system must spray it correctly. Deposits in the decomposition tube/mixer, a restricted line, or a weak pump can reduce effective dosing.
Signs:
- White crusty deposits around the doser or pipe
- “Dosing quantity” not tracking commanded rate
- Aftertreatment temps not behaving normally during events
5) SCR catalyst efficiency loss or damage
Catalysts can degrade from overheating, contamination, or long-term exposure to poor dosing. If the SCR brick is damaged, the system may never achieve the expected conversion, even with correct DEF and good sensors.
Clues:
- Everything else checks out (DEF quality OK, no leaks, sensors verified), but conversion stays low
- Temperature behavior suggests poor reaction efficiency
6) Wiring, connectors, and power/ground to sensors and aftertreatment components
Even though FMI 31 isn’t “open circuit,” wiring problems still matter—especially intermittent connection resistance, corrosion, or weak grounds affecting sensor heater performance or signal integrity.
Check for:
- Rubbed harness sections near the frame/aftertreatment
- Loose pins, water intrusion, and green corrosion
- Melted the loom near the hot sections of the exhaust
How to Solve SPN 4364 FMI 31?
Here’s a practical, shop-friendly workflow we use to reduce guesswork. The goal is to confirm whether you have a real SCR performance problem or a measurement problem.
Step 1: Pull all active and inactive codes, then look at “who came first.”
Don’t isolate SPN 4364 FMI 31 without checking the rest of the fault list. Codes for NOx sensor heater, DEF dosing, temperature sensors, or regen issues often point directly to the root cause.
Tip: If you have multiple aftertreatment faults, fix the “enabler” faults first (temps, dosing, sensor power), then retest.
Step 2: Check live data (NOx upstream vs downstream, temps, and dosing)
With a diagnostic tool, monitor:
- Upstream NOx (before SCR)
- Downstream NOx (after SCR)
- SCR inlet/outlet temps (where available)
- Commanded vs actual dosing (if supported)
Healthy trend (general): downstream NOx should usually be meaningfully lower than upstream once temps are in the effective window and dosing is active. If both sensors show nearly the same number under steady conditions, either conversion is low, or the sensors are lying.
Step 3: Inspect for exhaust leaks and physical damage
Do a hands-on inspection:
- Look for soot trails at joints
- Check clamps and flex sections
- Inspect sensor bungs and harness routing
A smoke test can help confirm small leaks you can’t see.
Step 4: Verify DEF quality and check for deposits
- Confirm DEF is fresh and at the correct concentration (use a DEF refractometer if available)
- Inspect tank cap sealing, pickup screen, DEF filter (if equipped)
- Inspect the injector area for crystals and restrictions
If you find heavy deposits, cleaning the mixer/decomposition tube and correcting the cause (poor spray pattern, low heat, wrong fluid) matters—otherwise deposits return.
Step 5: Validate NOx sensors (and replace if readings are biased)
If live data is slow, erratic, or “stuck,” or if swapping sensors (where design allows) changes behavior, replacement is often the right call.
For compatible aftermarket replacements, we stock a broad range of NOx sensors across many heavy equipment brands and engine platforms—built to restore stable readings at an affordable price.
Step 6: Run the correct test procedure and re-check after repair
After repairs:
- Clear faults
- Perform the manufacturer’s verification routine (often includes an SCR efficiency or inducement reset procedure)
- Run the machine through a full warm-up and a working load window
If your service tool supports it, an “override”/test mode during a regen event can help you watch NOx conversion behavior in real time.
Quick troubleshooting table
| What you see | Most likely causes | What we check first | Common fix |
|---|---|---|---|
| Derate + code returns under load | Exhaust leak, weak dosing, biased NOx sensor | Live NOx + leak inspection | Repair leak / clean doser path / replace NOx sensor |
| Downstream NOx stays high all the time | No/low DEF dosing, catalyst issue | DEF quality + dosing command | Correct DEF + repair dosing system |
| NOx readings jump or freeze | Sensor/harness issue | Sensor power/ground, connector pins | Repair wiring or replace the sensor |
| Heavy white deposits at injector pipe | Poor spray/heat, wrong DEF, chronic short trips | Visual inspection + DEF test | Clean deposits + correct root cause |
| “Everything checks out,” but the conversion is low | SCR catalyst degraded | Confirm temps, sensors, and dosing are correct | Replace SCR catalyst (as needed) |
Where parts shopping fits
If your troubleshooting points to emissions components, we recommend sourcing replacements by exact application and cross-referencing part numbers. These categories help you narrow fast:
- NOx sensors (common root cause for efficiency complaints)
- Exhaust system parts (clamps, emission/exhaust components tied to leaks and restrictions)
- For machines in the ag category, you can also browse tractor parts and then filter into related systems (engine, electrical, and aftertreatment-adjacent items)
As an aftermarket parts supplier, we focus on high-quality products at affordable prices, with a vast inventory and wide compatibility across many heavy equipment brands—so you can fix the root cause instead of patching symptoms.
Preventive Maintenance Tips
Preventing SPN 4364 FMI 31 is mostly about keeping sensor feedback trustworthy and keeping the SCR path clean and sealed. We recommend these habits for off-road fleets and owner-operators:
1. Use clean DEF and store it correctly
Keep containers sealed, avoid heat exposure, and don’t top off with unknown fluid.
2. Inspect harness routing near hot exhaust sections
Look for melted loom, rubbing points, and loose connector locks.
3. Check for leaks during routine service
A 5-minute soot-trail inspection around clamps and flex joints can prevent weeks of chasing efficiency faults.
4. Don’t ignore early warning signs
If DEF use changes suddenly or regens become frequent, treat it like a “pre-code” indicator.
5. Keep aftertreatment components free of buildup
Address crystal deposits early—once a mixer is heavily restricted, efficiency faults become much harder to stop.
Conclusion
SPN 4364 FMI 31 usually points to SCR efficiency not meeting target—caused by NOx sensor drift, exhaust leaks, DEF quality/dosing problems, or catalyst performance issues. If we verify live data, check for leaks and deposits, and confirm sensor behavior before replacing major components, we can fix it faster and avoid repeat derates. When parts are needed, FridayParts offers wide-fit aftermarket options at practical prices to keep off-road machines reliable.
