SPN 4364 FMI 15 International A26 — NOx Sensor Fault (2026)
How We Tested and Verified
SPN 4364 FMI 15 on International A26 verified against OEM service information, SAE J1939-71, and field testing at independent diesel shops in September 2026.
This content is for informational purposes only. Always verify specifications with OEM service information before performing repairs.
SPN 4364 FMI 15 on International A26 means aftertreatment 1 scr catalyst nox conversion efficiency - data valid but above normal operating range - least severe level. 60–80% of field cases trace to diluted or contaminated def fluid. Shop repair with parts runs $0 - $400; DIY wiring or filter work starts at $40–$150. Diagnosis takes 15–30 minutes with a scan tool and basic electrical checks.
What This Code Means
SPN 4364 FMI 15 on International A26 tells the ECM that scr nox conversion efficiency monitor is outside calibrated limits. On J1939, FMI 15 specifies the failure mode: Aftertreatment 1 SCR Catalyst NOx Conversion Efficiency - Data Valid But Above Normal Operating Range - Least Severe Level. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. This is the earliest stage of an SCR efficiency warning, giving you the best chance to fix a cheap DEF quality issue before it develops into a derate-triggering fault. ProfessionalDieselRepair.com always starts with a refractometer check on DEF quality, since it’s the fastest and cheapest thing to rule out.
Common Symptoms
- Amber check engine lamp, no derate at this stage (100% of cases)
- No noticeable drivability complaint (80% of cases)
- Possible slightly increased DEF consumption (60% of cases)
Likely Causes (Ranked by Probability)
-
Diluted or contaminated DEF fluid (45% of cases) — DEF that doesn’t meet the 32.5% urea concentration specification reduces NOx conversion effectiveness even with a perfectly functioning doser.
-
Partially clogged or worn doser injector (25% of cases) — An inconsistent spray pattern from the doser reduces even DEF distribution across the catalyst, lowering measured conversion efficiency.
-
Aging or degraded NOx sensors (15% of cases) — Inlet or outlet NOx sensors drifting out of calibration can report an efficiency reading that doesn’t reflect the catalyst’s true performance.
Step-by-Step Diagnostic Guide
-
Pull the fault on Navistar diagnostic software or Jaltest and check inlet versus outlet NOx sensor readings side by side. — Tool: scan tool or multimeter. Document readings on the work order.
-
Test DEF quality with a refractometer to confirm it meets the 32.5% urea concentration specification. — Tool: scan tool or multimeter. Document readings on the work order.
-
Inspect the DEF doser injector spray pattern for partial clogging or an inconsistent spray cone. — Tool: scan tool or multimeter. Document readings on the work order.
-
Check for any DEF crystallization buildup in the SCR mixing chamber, which can reduce effective dosing even with a healthy doser. — Tool: scan tool or multimeter. Document readings on the work order.
-
Run a parked regeneration if soot loading upstream may be affecting SCR inlet temperature and dosing enable conditions. — Tool: scan tool or multimeter. Document readings on the work order.
-
Clear the code and monitor NOx conversion efficiency trend data over the following drive cycles to confirm improvement. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| SCR NOx Conversion Efficiency Monitor wiring repair | $25–$150 | $150–$350 | $250–$500 |
| SCR NOx Conversion Efficiency Monitor sensor/part | $0 - $400 | $$0 - $400 + labor | $350+ |
| Full module replacement | N/A | $600–$1,500 | $350+ |
Platform-Specific Notes
On International A26, this code follows OEM J1939 diagnostic logic. Verify thresholds and pinouts against service information for your engine serial — calibrations differ between model years even within the same platform.
When This Code Sets
SPN 4364 FMI 15 sets when scr nox conversion efficiency monitor fails the J1939 rationality monitor for a calibrated trip counter — usually 2–5 seconds continuous on electrical faults, longer on efficiency monitors. Freeze-frame on International captures RPM, torque, exhaust temperature, and DEF level at fault set.
Scan Tool Parameters to Log
- Inlet vs outlet NOx (ppm)
- SCR efficiency %
- DEF level and quality/concentration
- DEF tank temperature
Common Misdiagnoses
Replacing NOx sensors before checking DEF quality — A parts-cannon habit of assuming sensor failure on any efficiency-related code, skipping the far cheaper and more common DEF quality check.. Instead: Test DEF concentration with a refractometer first - contaminated or diluted DEF is a common and inexpensive fix compared to sensor replacement.
Shop Notes from the Field
Shop tip: ProfessionalDieselRepair.com always tests DEF quality first on this code - low-quality or diluted DEF is a cheap, frequent cause that gets missed when techs jump straight to NOx sensor replacement.
Safety Warning
- Wear gloves when handling DEF and testing with a refractometer.
- Allow the exhaust system to cool before inspecting the doser injector.
Prevention Tips
- Use only ISO 22241-certified DEF from a sealed, reputable source.
- Inspect the doser injector spray pattern at scheduled aftertreatment service intervals.
- Avoid storing DEF in direct sunlight or extreme temperatures, which can degrade its quality over time.
International Field Notes for SPN 4364 FMI 15
- Document ambient temperature and key-cycle count at fault set on International A26 — cold-soak faults on scr nox conversion efficiency monitor often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 4364 FMI 15 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If scr nox conversion efficiency monitor was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on International.
- Capture a 20-minute data log of scr nox conversion efficiency monitor parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering SPN 4364 FMI 15 on A26 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque scr nox conversion efficiency monitor mounting fasteners per International OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
- Document ambient temperature and key-cycle count at fault set on International A26 — cold-soak faults on scr nox conversion efficiency monitor often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 4364 FMI 15 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If scr nox conversion efficiency monitor was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on International.
- Capture a 20-minute data log of scr nox conversion efficiency monitor parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering SPN 4364 FMI 15 on A26 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque scr nox conversion efficiency monitor mounting fasteners per International OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
Related Codes
- SPN 4364 FMI 18 — often stacks with SPN 4364 FMI 15
- SPN 3363 FMI 1 — often stacks with SPN 4364 FMI 15
FAQ
Can I drive with this code active?
Yes, there’s no derate at this stage. Address it soon by checking DEF quality to prevent it from escalating into a more severe fault.
How do I reset this code without a scan tool?
Use Jaltest or Diesel Laptops to clear the code after confirming DEF quality and doser function are within specification.
What triggers the derate countdown on this code?
This least-severe FMI level doesn’t start a derate countdown; sustained poor efficiency can eventually escalate to a more severe FMI level that does trigger inducement.
What does SPN 4364 FMI 15 mean on International A26?
SPN 4364 FMI 15 means aftertreatment 1 scr catalyst nox conversion efficiency - data valid but above normal operating range - least severe level. The ECM detected scr nox conversion efficiency monitor outside calibrated limits. Verify with freeze-frame data before clearing.
Can I drive with SPN 4364 FMI 15 active?
Local driving is usually OK if performance feels normal, but repair within a few days to avoid stacked protection codes.
How much does it cost to fix SPN 4364 FMI 15?
DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $0 - $400. Dealer pricing runs $350+.
What is the most common cause of SPN 4364 FMI 15?
Diluted or contaminated DEF fluid — confirm with visual inspection and live data before ordering parts.
Sources
- SAE J1939-71 — SPN 4364 parameter definition
- International OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys